Domain Map - make the drop down boxes combos instead?

Right now, for Domain Map, if you load domains, if either source or target does not have domains, you get an error pop up message. I think we need to handle single sided domain in either Domain Map or Value Map. The logic is, my target has a domain, but my source does not, but I know the values of source that I want to load into a domain of the target.

This is where a combo box would help. If one side does not have a domain, I can type in the value that I want to translate to a domain on the other side.