Populate main form list box thorough pop-up form check boxes?

[forceflow]

I would like to have users click on command button to open pop up form (no problem). Pop-up form consists of 20 or so selections that can be checked. On exit, I would like text representing each checked box to appear in a list box on the main form. This needs to be saved for each record so that when the record is recalled, the list box represents the items originally checked. Any ideas how to go about this "cleanly" (brute force will be last resort!). I considered doing this through a multi select combo box but thought it would be easier for users to use a pop-up since they can see all choices at once. Thanks!