No such thing as a dual-suspension kit, but Cane Creek (I think it was them) used to sell a pretty darn nice suspension seatpost.

On the frame swap thing, I did several of those during a period of stability in standards as my son was growing up. These days you really need to have a specific target bike, and then go through part-by-part to see what's possible to move across. If wanting to move your 29er wheels and tires, then you'll need a 29er frame and fork with the same dropout configuration as on your current bike.