Hi Gunnar,
the exit is ok to use but i'ts not possible to update LTAP (in memory or on DB) in there directly. As this exit is executed in UPD task at end of in memory TO creation, the data of TO is NOT written to DB at this stage, so a UPDATE ltap SET field=... WHERE lgnum= AND tanum will not work.
You need a own function module also capable of running in UPD task which is called in this exit with the TO number. This FM executed after the SAP TO creation can do the UPDATE on DB mentioned before
Best regards,
Matthias