Can you use the bottom chord of a roof truss to support a attic deck? Does the chord have to been made of a truss joist?
If @freeman would like to make his comment an answer, I'll be happy to upvote it and (probably) delete this one.
Generally, roof trusses are made as a system of lightweight materials that together function to hold things up. Most of the time, they aren't designed to have things weighting the bottom chord. Pushing downward on a truss that's not intended for that force will end very badly.
So, you probably can't.
However, if you bring an engineer onsite, he or she can tell you whether I'm right, and more importantly, could design something that would carry a load.