To do this you would need to cut a new hole in your blank tank and patch the old one. It might be easier to get a new black tank. Better still would be to switch to a composting toilet so you don't need a black tack at all, and the old black tank gets used as a gray tank. This would mean you could simply use the old toilet flange for your shower drain (after thoroughly cleaning out the black tank, of course!). Like many others, I've found the composting toilet solution to be far superior to wasting water, carrying around a huge tank of ____, and dealing with dump stations!
25' 1972 Airstream Trade Wind Land Yacht (heavily customized)
2006 Jeep Liberty CRD (diesel)