After playing around for a bit, I don't think you can.

I'm sure you are already aware, you can go to the icon under Actions and Traits and set it directly. However, as I suspect this is part of something bigger and likely more complex, it is worth mentioning that you can copy whole crossbars, or just individual ones, so, you could put it somewhere that you don't use, like a class cross bar, and copy it into place from there.