Or create another lookup on the Case form for Opportunity and toggle the Customer and Opportunity lookup using a radio button field, that when the user clicks on Customer, let him choose from the Customer look and when he/she clicks Opportunity, disable
the customer lookup and open Opporunity lookup to choose from.
I don't know what exactly it is you are trying to achieve, but if it helps!
Best Regards,
Gagandeep Singh
http://mscrmnovice.blogspot.com