Trying to add an extra cog I am trying to use a multigear bike to make a pedal power bike. I am trying to add an extra cog to attach a chain to connect the motor to. See link.http://www.popularmechanics.com/technology/gadgets/how-to/a10245/pedal-power-how-to-build-a-bike-generator-16627209/
Is it possible with multigear or do I need a fixed gear?
You need a rear hub that can have a cog on both sides.

Some fixed gear hubs have threads on both sides, or a fixed thread on one side and freewheel thread on the other. The author apparently has one of these.

The multi-gear option is to use a disc hub and cog that has been drilled to fit 6-bolt disc brake mount. This trick is usually used to make fixed gear hubs out of MTB front hubs, but could also work here.