If I understand your question correctly, you wish to find a way to implement D&D 5th edition class archetypes, as a type of "prestige kits" in AD&D 2nd edition. Great idea! I think before considering the prestige aspect of the question, it will be helpful to express the class path as abilities with point costs like the ones in Skills & Powers. Here is how we might start:
Open the 5th edition class you wish to convert and find a class in the Skills & Powers that you think is the most similar
Take some time to read through the Skills & Powers entries, considering what kinds of abilities are appropriate for the class, and how an ability's power is reflected by its point cost.
Go through the 5th edition class path and assign point values to each archetype feature. You will have to make judgment calls here. Always ask yourself "How will this power affect that balance of the class? How powerful is this ability compared to others?" A good rule of thumb might be: if you cannot think of a reason NOT to take the feature at the point cost you have assigned it, you may want to make it more expensive. Similarly, if a player is spending so many of his/her points on the feature that it will hinder their ability to take basic class features, it may be too expensive. NOTE: some abilities in 5th edition simply are not appropriate for AD&D. Do not be afraid to say "no" to an ability, or better yet, change it so keeps the flavor but fits better with the mechanics.
Do a mini play-test (sort of). Add all of the features you have converted and given point costs to the ones already available in Skills & Powers (make a printout or something). Tell a few of your players who are familiar with AD&D 2nd edition to make what they consider to be the most powerful character they can of the class you have been modifying. Make sure they all have the same number of points to spend, and try to standardize the test. After they have selected class features, take a look at the ones they chose. Did they ditch all the originals for the new ones? If so, they may need to be a little more expensive. Did your hard work go unused? Maybe try to incentivise it with lower point costs. If you managed to get a good mix between old and new you probably did a good job.
Now that we have the conversion method outlined let us go over an example using barbarians.
Open to page 49 of your trusty 5th edition PHB. In AD&D 2nd edition, barbarians were more or less a kit for warrior, so I am going to use that as our AD&D benchmark which means the beginning of chapter 4 of Skills & Powers.
Read the warrior powers. Reading some of the racial powers can also be helpful.
Assign point costs. I am not going to assign point costs to every barbarian ability, or we will be here all day, but here is how you might approximate the classic barbarian rage:
Rage is barbarian's bread and butter. It gives you a damage bonus, as well as significant defensive bonuses. The damage bonus for 5th edition (+2@lvl 1 to +4@lvl 20 is on par with weapon specialization in AD&D (flat +2), so the cost of weapon specialization is a good place to start. Keep in mind that weapon specialization in AD&D also comes with a bonus to hit and more attacks per round. We are only getting the damage bonus here, so to balance it out, you may want to keep the idea from 5th edition that the bonus will scale with level. Rage is at 5 points and counting.
Resistance to bludgeoning, piercing, and slashing is pretty big. The closest approximation is the "Dense Skin" feature for dwarves which costs 10 points! Adding another 30 points to rage is just not valid, meaning this feature needs to be reworked for AD&D. 3.5's damage reduction comes to mind and may be a good option here. Perhaps a flat reduction of 5 points of damage off of every physical attack would work better. You could even take this a step further and add one point of DR for every extra point you spend on this ability. 5 points of DR means 5 more points (we are now at 10)
Advantage on Strength checks and saves is relatively easy to imitate. You might treat the barbarian's Strength score as 4 points higher for the purposes of bend bars, lift gates, open doors, etc. As far as saves go, you might want to let the player choose one save they will get a +3 bonus on for the duration of the rage. A permanent saving through bonus for dwarves is 10 points, but always active. This bonus will only apply during rage, so 5 points sounds good (now at 15 points).
When it comes to DURATION and DAILY USES, there are A LOT of options. A static duration might be too restricting, whereas a variable one could quickly get out of control. Fewer daily uses can balance a longer duration, and a tricky balance must be stuck here. For a ball-park estimate, you might say the player gets one rage per day, but it lasts 10 rounds. Another option might be the player gets rounds per day of rage equal to their constitution score, divided however they wish (but can only start and end rage once per encounter). The final point cost here will be quite variable depending on how you choose to deal with this. Going with the rounds = con score option, I am going to label this one at 5 points, since scores in AD&D tend not to be that crazy high.
and our FINAL TOTAL IS...
CRIES OF OUTRAGE
"BUT WAIT THE FIGHTER ONLY STARTS WITH 15 POINTS!!!!!"
Yes. That is true. But consider this: you can carry in 5 points from your race and get it as it is laid out here, or, you can just not take some of the features, and have a less potent rage. The great thing about Skills & Powers was that it made the classes modular, down the feature level. You could spread your points or blow them all on one thing if you wanted.
Whew... I think that is enough for the conversion part. Concerning your idea of making these paths prestige kits of sorts:
Assuming you are going with the Skills & Powers rules for leveling up where the players gain character points for every new level they gain, you might want to set an "entry fee" of character points for the privilege of taking the new class features. Beyond that, the characters would have to scrimp and save their CP to gain the features you have laid out, or improve ones they already have (maybe your bare-bones rage will get stronger as you spend more CP on it?).
Anyway, that is my 2 cents. I know I can be rather verbose and I truly appreciate it if you made it this far. Thanks.