[otrs] Custom customer field - drop down

I was wondering if there was a way to create a custom field for Customers that is a drop down list? We need a field for Office Names, but I'd rather not leave it as free text since we'll be generating reports based on that field. I got another regular custom field for the Phone number working like the documentation said, but can't find anything about making a drop down. I saw that the "Valid" field is a drop down and that it's pulling it's data from the "valid" table based on a key number in the customer_user table. Having something like that would be perfect. Thanks in advance for your help! Keith