So in 4e, spellcasters use focuses like warriors use weapons: every spellcaster has to have one, and they can be magical and impart bonuses on attacks made with them (and everything that affects foes is an “attack” in 4e).
In 3.5e and Pathfinder, spellcasters do not have any “weapon” equivalent the way focuses work for 4e spellcasters. Instead, spells are completely independent and just work. They can’t get these bonuses, but are also not supposed to need them, so it’s often an advantage (no need to spend money on a weapon).
Ultimately, I think the 4e approach is superior; I think that parallelism helps keep warriors and spellcasters in similar boats. It’s also pretty fun! You get more options and more ways to customize your character.
But since 3.5e and Pathfinder spells are designed around there not being such bonuses, adding those on top is problematic. If nothing else, they have to cost a whole lot more than weapons do: consider the chasuble of fell power in Complete Arcane, which costs 8,000 gp to add +1d6 damage to eldritch blast or 18,000 gp to add +2d6 to eldritch blast. Adding +1d6 or +2d6, to a single invocation, is costing approximately what a +2 or +3 weapon costs.
Now, warlocks are underpowered, and the chasuble of fell power is arguably overpriced (at least excepting when you use to qualify for things, which might justify its cost). But other spellcasters are very powerful. If you are doing this as 4e does—making it a part of spellcasting in general, invocations included—you are going to have to rebalance every magical effect in the game. That is, clearly, not anything like easy. You’d be better off throwing them all out and starting from scratch, maybe using the originals for inspiration at best but nothing more.
And if you seriously do that, you’re going to be reinventing the wheel a fair bit, because that sounds a whole lot like you’re just recreating 4e. Better to just play 4e at that point.
If you do this as a warlock-only thing, it’s probably easier. They’re lower power, so you can probably afford to give them straight-up bonuses without imbalancing the game overall. It’s still not easy. Particularly since 3.5e invocations are much more varied than 4e powers (which could be relied upon to require an attack roll and probably deal some amount of damage, which were things you could give standardized bonuses to). And especially since part of the goal is to have a variety of different focuses you might use, all of which has to be considered against all of the invocations.
So no, there is no “easy way” to handle this. It is going to be an extended, involved project, probably requiring playtesting and several iterations.