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I have pre made truss ceiling joist in the garage. The joists are 2x4x21'.
I want to use space above the joists for storage. I want to sister a 2x6x21' or 2x8x21' next to 2x4x21' is this ok?
If it is ok how do I do this: nails, bolts? Any other recommendations? Or should I put the new lumber in between existing joists?
First you need to determine the weight of the items you plan to store. If the trusses were designed under the IBC/IRC or the UBC (and maybe the BOCA and SBCCI) codes, then they should of been designed with a 10 psf live load on the bottom chord. If you keep below this loading you should be okay, but you might want to check out the code requirements for your building. 1/2" plywood will add a 1.5 psf load. A 2x10 at 24" o.c. will add 2 psf (but a 2x4 will add .75 psf) to the ceiling. A 2x12 at 24" o.c. will add 2.5 psf. Subtract the plywood and add-on from the 10 psf to get what load you can add. The safe bet is to consult with a structural engineer familiar with your truss assembly. Could prove to be the lesser alternative to a truss collapse.
It sounds like you need to support the 2x6 or 2x8 from below with a load bearing wall or beam. If you will be walking around up there, then you should build it like floor not a shelf. Nailing floor joists to the ceiling trusses would probably be slightly less optimal than not nailing them together because flexibility of the joist is part of the strength, but I doubt it would make a huge difference. If you decide to sister 2x8 floor joists to 2x4 trusses, nails would be best.
