Custom Attributes filtered on department

Custom Attributes filtered on department

Hi,
 
We have a situation where we want each user to be a member of 1 to 3 departments, for Example a Modern Foreign languages teacher who taught French and Spanish would be a member of  the 3 departments "Modern Foreign Languages", "French" and "Spanish"
 
I have successfully created custom attributes for Department two and Department three, what I now want to do is to set them to filter on "departments" a the primary department attribute does when creating or modifying users. This is to ensure conformity and that only genuine departments are used, can you advise if this is possible and if so how it can be done please.
 
Kind regards,
 
Matt

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