GL Master - Alternative account number validation

We want to use the alternative account field on the GL master (transaction FS00) and only be able to pick from a COA in the system, the help text states this is both a freely definable field and you can choose from an existing COA.

Is there any validation/user exit that can be setup so you that you can only choose from an existing COA and not input any text in this field?

Thanks

Joe