Transforming a passenger airliner to a freighter seems to be common. If I understand correctly (do not hesitate to correct me), it basically consists of removing many components (seats, galley, ...) as done in the first phases of a D-check, and then the handling equipment is added (as it should be done in the late phases of a D-check for freighters).
How much does the transformation cost compared to a D-check?
Related: How much does a D-check cost compared to a new aircraft? (Useful to have an idea of D-check costs.)