Hi,
The import purchase can be well identified at header level itself by vendor number (and vendor country) and usually there will be a different pricing procedure for import purchase if you are supporting Indian client. In this case, you can very well propose a different document type for import purchase and maintain a different field selection group for the new document type. Maintain the net price field as noneditable for the new field selection group.
If its a different pricing procedure and different gross price condition type, then you can even maintain the gross price condition type as "no changes possible" in M/06 as shown below:
Please test the scenario and revert back. If required, you may go for a different condition type for gross price by copying PB00 also.
If you still want to proceed with same document type, then you have to check the feasibility of development. You may check whether a custom routine in VOFM (and assigned to the gross price cond type in pricing procedure) can fulfill your requirement. Please check the same with technical team.
Regards,
AKPT