Edit or supplement Maps?

Is it possible to add missing roads to the maps?

I do a bit of hiking, and Garmin already has a lot of the forest roads, but not all of them. A couple times the roads I've needed are missing.

Can you add them in with the PC software and have them show-up on your Nuvi?

Would I be better off just getting the topo maps on a SD chip from Gramin. (Has anyone tried them on a Nuvi?)

Thanks