This is great in a fantasy or a porno, in real life, it'll suck in every way but the way the OP was hoping for.
There's so much that can go wrong here, even if she signs some type of consensual clause and an NDA they still can make your life miserable when she underperforms in the office or in the bedroom.
(just as Trump how that NDA stuff worked out for him with Stormy Daniels. Paid a hooker $130k and she complained about his mushroom sized dick
).
Just imagine the consequences when she stops answering the phones or caring for her professional duties, but gives great head! LOL, what will you do with her?
Or worse, she stops caring for her professional responsibilities, and now she started to use a lot of teeth when giving a BJ, or pulls the old, "I'm tired, are you cumming yet?" <nagging voice>
It'll be tough to fire her. Because she'll have the goods on you either way.