Hershel,
There is not a way to convert from a Hot Desk User to a Standard Phone Device. Although you can delete the Hot Desk User and the mailbox will remainl. Then you can create an new extension with the Hot Desk Users extension number and it will ask if you want to associate them or not. At this point I would just copy and paste another desk phone that works the way you want it to and has a mailbox associated with it.
Thanks,
TE