You won't be able to do it with a single CC (like a vba userform listbox set to multi-select).
But you can either do a series of checkboxes or make it a dropdown CC inside a repeating CC.
See this to understand the concept -
Word 2019 and 365 Tutorial Repeating Section Content Control Microsoft Training - YouTube. You can choose to include only one CC in the repeating CC.
I think this will be far simpler to set up and probably simpler for authors to use than going down the vba userform wormhole. If you are using a vba userform already for other things then it might make more sense to continue that way so the users don't have to mix and match their input methods.