Possible to make a selection in a subform from the mainform combobox?

[graccess]

Trying to find a workaround for another problem and so I'm resorting to adding a new combobox in my mainform which will add a value to the combo box in my subform.

To clarify, I'm choosing a primary company in my main form from a combo box called 'primary.' In my subform, I have combo box for related companies, let's call it 'related', where I have multiple selections made and I'd like the selection that is made in the main form to be included in those selections. Hopefully that made sense. Not sure if this is even doable from a programming standpoint.
Thanks.

[graccess]

One impt point to clarify here, both combo boxes are referencing the same table, tblCompany. The difference is my [primary]combo box is unbound and the row source is referencing the tblCompany table while the [related] combo box is bound to the tblCompany table. I'm not trying to filter the [related] combo box, only wish to add a selection to the list in that subform control.