Custom Field dynamic values

Custom Field dynamic values

We are looking to automate user creation using servicedesk pro.  We will have a request template for new hires that will be processed using the API's.

Is there a way to make a custom field that will display all existing users departments?
Or is there a way with the api to add and remove custom field values?

I know i could do this directly in the database but for compatibility reasons I am only preforming read operations to get a list of Tickets that match the criteria to be processed.