If the combobox is populated using RowSource, then the code behind the
command button would have to resize the row source range and add the new
folder to the rowsource range.

If the combobox is populated during the initialize event using the add item
method, then the command button code can use the add item method to add the
new folder to the combo box list as below.

Private Sub CommandButton1_Click()
ComboBox1.AddItem Selection
End Sub

This assumes the user will have typed a new folder name into a cell and
then selects that cell.