There's no button for that, but in theory you could. Click on the person you would like to create a servant for, then "add a partner" (or ex-partner). Change the type to "other", and type in "servant". It should say something like Partner: Tim (Servant).
Note that, as far as I know, there's no way to make it not reciprocal, so the other person will also display as Tim's servant.
Does that answer your question?
