sure you can add gearing to a single speed cruiser, throw on an Internal Geared Hub. I run a shimano nexus 3 speed and its pretty nice...they make a 7 speed as well. Or you can go cheaper and get a Sturmey Archer IGH...
I suppose you could also throw on a freehub with a cassette and spread out the rear forks to make it fit and then drill out some holes for a dérailleur, but that could be difficult to adjust.
On the other end you could also change the crankset and put a triple chainring on the front... front dérailleurs are comparatively easy to put on a bike thats never had em.
how about some pics of your project?