Yes, there's the "ML ninjas" program, which brings non-ML SWEs into brain and other groups for a deep dive into machine learning through a project-based approach. (Often, but not exclusively, inspired by a problem from their originating area.). It's not a transition per se, but a great outcome is that the person becomes a bridge for their team between their specific domain of expertise and machine learning.

A full transition would be done by the usual transfer mechanisms. Two of my former students have switched into Brain from elsewhere in Google, for example.