CRM 2011: Changing a USER Look-up field to allow CONTACTS as well

Greetings,

I currently run CRM 2011 on premise. I have a Custom Entity calledTraining Sessions with a look-up field calledInstructor.

The Look-up field was originally linked to the USERS entity; our instructors were originallyemployees and CRM users.

I have now been tasked to make a change to this existing Look-up field to allow a user to select aUser OR aContact as we now have instructors from Third-party companies who are NOT CRM Users.

Is this possible to achieve?

How can I go about achieving this WITH-OUT code?

I am looking for a solution that does not require development, just configuration.

Thanks,