Is it possible to enter the referring provider and last date they saw the patient on a claim?

Yes, you can select a referring provider on the encounter entry screen and enter the “Date last seen”. 

The "Date last seen" is entered on the patient's case. Case tab > Condition tab > on the Date Type select from the drop down "Date Last Seen" and then enter in a start date.

These steps will populate the claims with this info in the standard place on the claim (referring provider = Box 17,  Date last seen = Box 14).