adding a center channel without a 5.1 amp
 

right now the setup goes through a DQXS (6 channel eq/cross)> jl xd600/6 but after spending some time in my mom's rdx with a center channel (5.1 setup) i really liked the sound and want to add that to my van would it work if i would come off the dqxs highpass with a Y connector to a 2xs with a small 2 ch amp, or possibly just a 2 ch amp without a 2xs

That will get you signal - it won't be a processed center channel mono signal though.

Are there any non discontinued processors that can add that functionality? There few I just searched on seems to be unavailable. :-$

Actual processed center channel? The most popular was the Audio Control ESP2. Only saw one on ebay.