I'm not sure about this, but I think that highlighting is something your browser knows about, but the server doesn't. If so, the simple answer would be 'no'. (You can see a similar problem in that smileys always get inserted at the end of the text, no matter where the cursor is - if you decide you want a smiley at the end of a sentence you're revising, you have to cut&paste)
On the other hand, a button that added [ b]your bold text goes here[ /b]
or something similar would be dead easy to provide.