Pretty exciting stuff, it's great to hear old voices and presences from the television show. New content like this amazes me how STO remains F2P, or even solvent. Buy those ships people, and perhaps next we'll get Patrick Stewart!
I don't know for certain but I think it probably doesn't cost them that much.
A great number of Star Trek actors have shown a willingness to participate in things for little (if any) financial compensation.
I imagine a number of them would be more than happy to do voice work for STO without charging through the roof for it, it is an opportunity to bring their characters back to life in front of millions of fans.