I don't see what the problem would be in changing to a suspension fork ... basicly you just take out the fork you have now and replace it with a light 700c suspension fork ... I have the suntour CR9v which I like quite a lot and is not too expensive.
I also don't see why"atx 6speed" says that your options are limited or that you need a 29-er fork ... you have a 700c rigid fork now so you basicly need a 700c suspension fork to replace it with.