Workflows: Processor to create associations between objects which are not enrolled

I have object types A, B and C.

Enrolling an object of type A in a workflow creates an object of type B. It passes on record_id of A and record_id of C.

I would like to enroll object of type B into a workflow to create an association between record_id of A (enrolled object) and record_id of C (const value).

Currently this requires a Custom Code action since you can only match values from an associated object to the record_id of the enrolled object.