I don't think it's possible directly in the field mapping step. As a workaround, you can temporarily assign a different conversion object type in step 1, change it back to the original one, then go through all steps again until the field mapping,
All previously definded source structures and fields should still be there, so it's just a matter of saving it again.
To be save, you should try this first with a copy of your LSMW object.
Thomas