not sure if this is what you mean but you could make the ability a normal ability and give it to the hero.
to do this just change the "stats - hero ability" to false and the "stats - item ability" to false in the object editor and it will become a normal ability which any unit can use
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.