For the Collapsible part
You could use the "Snap baton" as a starter guide. Another that is "Extendable" is the MSW-9 molecular stiletto. It goes to 30 cm from a data pen. its fragile due to the makeup but also due to thin size of extension. So a thicker one may hold up better.
So when it comes to collapsing staff down to size, you have ideas in the arms book but also its been done in real life with batons and other weapons. Just increase size.
For Attaching the Saber to the end
For the construction just add a screw on top for the saber. Shorten the lightsaber length in the build of saber. Next extend the bottom of saber with a "Screw" type pin that inserts/twists/slips into the top. At this point you will have your "Lance Staff".
In your and your DMs shoe's however, I'd just use a Force Pike and add a "Holder" near the tip for the saber. Those force pikes at full setting can cut thin dura-steel. Also, you could always have one side the force pike and the other the saber and try to give it the mod of Snap Baton in the middle. If you id say the center was collapsable and the tip of Force Pike and The saber were the only two that did not collapse. So a bit longer. The saber can be removed and the FP shortens a bit. Up to DM.
If DM Allows you could add a screw/magnetic/slip connector to the bottom and have a Denn'bok attached to the bottom. While the Denn'bok is "Babylon 5" gear... if you are also trying to go homebrew but just asking for a build procedure using books for numbers, I wouldn't see why this can't be added. It only has stats of a staff and easy to just say it can only power up when attached to the saber using its power as a power source through the same connector. Remove it from saber and keep it small or extend it and remove saber for more "Slip by authority figure hunting the Jedi" but still have a basic staff.
With the exception of the Denn'bok I kept to the Arms Guide with mod-tweaks but also remember the books are guides for ideas. All of these weapons at one point were not written down and someone decided an item/object/whatever was cool and did a writeup. All core books will say they are guides to help get you going. In the end, its what you do that makes the game. With DM approval of course :) ;)