In my head canon for "The Rocky Horror Show Show," Frank has rigged Eddie, along with several of his former consort and victims, with animatronics and every so often activates them like Chuck E Cheese statues for musical numbers. I think I saw the Cee-Lo/Rockafire crossover and internalized the idea. So you could probably get a singer for the role, if not an actor: maybe Jack Black, who certainly loves to do singing voiceovers for shows.
As for Rocky, I go back and forth between him as a spoken and as a mostly-mute character on television. He would probably be more of a cameo than anything, just one of Frank's experiments that didn't turn out QUITE right.
I changed my mind- Tara Strong can do Magenta- Amanda Palmer would be fun but I don't think she's that variable of an actress. And for Columbia, Yael Stone of "Orange is the New Black."
FRANK: "It's only a simple body transferrence, Brad. What's the worst that could happen?"
BRAD: (glasses pull) "What's the worst that could happen? God dammit, don't you remember when you built that sex android? Everyone was killed and the house blew up! Have you forgotten already?"
FRANK: (after a beat) "Yes. Throw Lever One, Columbia!"
(Columbia's hand hovers over a lever marked "Lever One.")
COLUMBIA: "Throwing Lever One, boss!"
FRANK: "Throw Lever Two, Magenta!"
(Magenta's hand hovers over a lever marked "Lever Two.")
MAGENTA: (to herself, mocking) "Throw Lever Two, Magenta."
FRANK: "Throw Lever Three, Riff Raff!"
(Riff's hand hovers over a lever marked "Lever Three," before moving to a different lever, marked in gothic typescript "F*CK SH*T UP.")
RIFF: "Hmm..."