So Frodo can use Brown Quest Cards (White hobbit cards in the old edition) as * cards. Can he do the same with brown Legendary cards (feature cards)?

A strict reading of the rules would say he can't

You are right.

I only have the original game so you may need to translate the terms but - Frodo can use any white Hobbit cards, with one symbol only, as *Joker cards.

(He can't use the non white cards in this way including Feature cards or those with more than one activity symbol on them as *Jokers).