Oops, I read your requirement wrong. To append the next value selected to the end of the list you will need a second RichText CC titled "List"
Code:
Option Explicit
Private Sub Document_ContentControlOnExit(ByVal ContentControl As ContentControl, Cancel As Boolean)
Dim oRng As Range
Select Case ContentControl.Title
Case "DDList"
Set oRng = ActiveDocument.SelectContentControlsByTitle("List").Item(1).Range
oRng.Collapse wdCollapseEnd
oRng.InsertAfter vbCr & ContentControl.Range.Text
End Select
lbl_Exit:
Exit Sub
End Sub
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(Pseudo Multi-Select Dropdown Content Controls