Using Nintex 2013

I have a Distribution Groups list. Each Distribution group can have one or many Parent Distribution Groups which can be selected on the form in a multi-value lookup . The lookup list is large so there are 2 associated fields for filtering ( Alias,Region). The form contains the same Distribution group multi-value lookup 3 times. The connected  Distribution group multi-value lookup is called existing and is unfiltered. The other two are filtered unconnected versions called alias and region . I want to update the existing lookup with selections from alias and region lookup. How can this be done in the form so that the connected lookup contains and saves all selections.