I think you can create a UI only model and a UI only field in that model of type Text. Then you a can pass in the value of a field from another model using an update field model action, global action, Button, or action sequence. You would have it grab the existing value of the UIOnlyField and append the new value and update the UIOnlyField with the combined. Something like this:

{{$UIOnlyModel.data.0.UIOnlyField}},{{SourceField}}