Installing subfloor - edges not hitting joists

Before going to far with this are you even sure the trusses are rated for top loading?
Is this going to now be a conditioned area? (HVAC)
By adding plywood your cutting way down on the avalible space for insulation.

To ansewer your question, yes you can sister some 2 X 6's.

Thanks. These are attic trusses rated for 40 psf LL. the attic space will be conditioned. Do i need to sister the 2x6 the entire length of the truss (from wall to wall) or can i add it just where i need it in the center section of the truss?

Called truss mfg. He says I can use a 2x4, just where I need it, nailed to the 2x6 truss with 1 16d nail per foot.

I'd be using a nail gun not a hammer, unless you want to do some sheetrock repairs.