The field MARA-EXTWG is not ready for input and was therefore not transferr

Trying to store vendor specific material group information via LSMW.

Getting following message and respectively the effect described in it:

<b>The field MARA-EXTWG is not ready for input and was therefore not transferred

</b>

Any suggestions on why and how to force it through ?

Or maybe I need to store my external material Group+Subgroup ( 3 + 3 chars ) somewhere else? We only require this information for further grouping of materials in reports...