Some SEs would gain more value from more experts, and subreddits usually have more users than a given SE.
For example — How can you convince users at r/ask_lawyers, r/auslaw, r/canadalaw, r/law, r/lawschool, r/uklaw, etc. to to use Law SE?
If you want to try to get folks to move (or augment) their participation in one community to another, try posting content from the other community in a way that encourages them to come over.
Share blog posts, good content, etc.
In short, you cannot make anyone move from one to another - but if you provide enough "good" content from the other to the one, you might garner some new users