Ok, I can test this out for myself in a few weeks, but maybe if you made a mummy die, brought them back and breeded with a vampire? Except I forgot that mummies can't try for babies but would that apply to their ghost? Man, I need to try that!!
EDIT: This is Mia Mummyturner!
She decided to turn Gunther Goth and Nancy Landgraab into mummies!
She then killed Gunther to get a ghost.
While they waited for that special phone call from the lab, Nancy practiced martial arts and trained her to help her get to Emperess of Evil.
When the phone call came, Gunther was restored as a mummy ghost!
He then asked Nancy to be his wife, and was able to woohoo but not try for baby. So he broke up with her, and turned his attention to Mia. He married her and was able to woohoo but not try for baby so I conclude that you can't breed mummies no matter what!
I can't wait to get LN to experiment with vampires!