Hello Zain
You can only add fields that are available in the Coding Block (structure COBL). If the field is available in COBL you can use the enhancement SAPLFMDT to read it and do the derivation using a rule of the type enhancement. You can also use BADI_FMDERIVE to code your own derivation.
You can use the temporary field USERTEMP1 to store the values you read. You can also extend the structure FMDERIVE with a customer include.
Thanks
Shyam