TOPIC: Making a default field a dropdown

If I wanted to modify one of the default fields so it was a dropdown vs. open text, is that fairly straightforward?

I think it's this line in dtregister.php that I'd need to modify but I'm not quite sure how:

My client has a set number of organizations that could be listed and wants to make sure users are choosing from a list vs. typing freely. I know I could do it as a custom field but then it's not where we want it in the form order, and won't display on the back-end Records view as easily.

??

Thanks.