I’m not sure where the managed package comes in, but the only way I can think to get an auto complete would be to: 1)create a custom object called mailing country. 2) Import a list of off the countries into it. 3) Then create a lookup relationship field to the mailing country object in your your contact object. This will display as an auto complete when you bring it into your Skuid page.

Raymond is on the right track.  And here is an extension. 

Since you have a managed package - you probably cannot change the text field to be a lookup to the new related list.  So what you can do is create a UI only reference field that does this lookup,  and then an Action Fwk sequence that fires on save that updates your text field with the name value from the reference field before saving the record. 

We have a wee bug with UI only reference fields right now that might make this awkward,  but we’re fixing it…