Cards from The Fellowship Sphere (let's say Treason of Saruman), can be used in other scenarios from other expansions?

Only ones where you have a Fellowship hero. So you can use a Treason of Saruman card in a Black Riders quest, for example.

But if you want to ignore this rule and put them in a deck for another quest and try to cheat them in by some trickery, give it a try. The fun police won't come for you.

Thanks!