Board Thread:Manga/@comment-34172004-20190226211310/@comment-1327106-20190228002641

It should be possible. In a Q&A Isayama was asked what would happen if the Ackerman and Reiss families had a kid together. His answer was that both bloodlines would be represented in the child. Since you can't become the Founding Titan without being turned into a pure Titan, we can extrapolate from Isayama's answer that there's nothing about being an Ackerman that prevents someone from becoming a Titan.

You probably wouldn't want to turn an Ackerman into a Titan though. Given that they were intentionally developed as guardians of the royal family and they placed crazy amounts of devotion and strength in a human-sized body, it's pretty obvious the Ackermans were designed for human-sized conflicts. That they happen to be good at fighting Titans was likely secondary since a ruling Founding Titan during Eldia's heyday would have been able to control Titans at will and would not have been in danger from them.