Hi Felix,
Best option is decrease the length of the custom fields less than or equal to 960.
I faced the same issue earlier which contains 2500 length(almost 50+) custom fields :). Since only 10 fields are required to be updated and those fields are available before 960th position, able to update the value perfectly. If fields are available before 960, try to update it, else another possible try to change the field position risky way and also make sure the field not missed during movement.
BAPIPAREX have only 4 VALUEPARTS and it is used in multiple place, If you try to do append the fields like VALUEPART5 and VALUEPART6 etc., some depends standard structure will get affect due to enhancement category not met.
Regards
Rajkumar Narasimman
