Allow multiple selections to display on form Rows?

Hi, we use a User Variable called "ProductSelector" to allow the user to make a selection from high up the hierarchy, and then in the Rows on the from we specify ILvl0Descendants(&ProductSelector). This allows the user to select a grouping in the User Variable at the top of the grid, and all the leaf members will appear on the grid.

The users now want to be able to make multiple selections, so that they all appear on the form. For example, select all the products grouped under "manufactured" as well as all the products grouped under "imported" and they all appear on the grid.