value set on validation type TABLE but allow override? possible?

Try to add new parameter on a copy of print selected invoice concurrent program.  The new parameter is assigned to a new value set called EMAIL_LIST_VALUE_SET.  This new EMAIL_LIST_VALUE_SET has validation type = TABLE since I will get a list of email from ra_customer tables.  That part is simple.  But the user requirement also requests if end user does not want any email address from the ra_customer table, user is allowed to manually type in any email address.  

How to do that?  EMAIL_LIST_VALUE_SET ( with validation type = 'TABLE' ) is not allowed to enter any value other than from the LOV.  What option do I have?  other validation type?