Hi Dhilipan,
I briefly checked the program code. In the API1006_HEAD structure, field BPARTNER is transferred to GTS with value <SPACE> (hard-coded in Function Module /SAPSLL/DEBMAS_1006_MAP_R3), and I think that might be the field you'd need to use.
From what I can see, that field is used as the Partner Number in GTS if it is filled; otherwise field EXTERN_NO is used. So it would be worth a try (in debug) to fill the BPARTNER field (max. 10 characters) and see what happens.
I'm not sure how the Number Range is assigned - it could involve some "profiles", or perhaps be a nice surprise. I'm sure you'll have fun in finding out: do please let us know.
Regards,
Dave