Please tell us a little more of your purpose, and current implementation, it might get you a better answer.
If you have the possibility to do the mapping in the receive port, then you can promote your field and set the orchestration variable in an expression shape, (or maybe assignment shape).
If you need your value back in the “original” schema, why not first map to a “copy” of it, maybe with one extra field to get some value, and then continue to map to next format.
Kind Regards
Martin Bring