Is it possible to make the Role lists that appear on the Credits and Musicians tabs editable, the same way such things as Category, Condition, Format, etc. are?

Also, I've noticed that sometimes these lists autofill as I type and other times not. It seems that you have to click on the down arrow at the right edge of the control once to "enable" autofill. Autofill is useful for these fields and I'd like it to work without the extra step if possible.