Prenith,
Considering your custom field is updated in MARC table and you want to update the same in free attribute field while CIF, use EXIT_SAPLCMAT_001 Core Interface to APO: Enhancements to Material with include ZXCIFU01.
Write your logic in this and map it to attribute field in APO, system will automatically push the values from ECC to APO every time CIF is triggered (for newly created and change materials).
Let me know if it resolved your issue.
Regards
Rahul