Hi Jaya,
Use this BADI BBP_BADI_EXTREQ_OUT to make any changes to the Purchase requisition that is being pushed from ECC to SRM. Trying setting that field based on conditions so that you can achieve your cause.
This BADI is called before the report BBP_EXTREQ_TRANSFER is executed(the report transfers the PRs from ecc to srm).
Mark answers helpful, because it helps in analyzing the problem. Many intellects comeout with various ideas, and ur marking helpful answers drives us in answering ur question to the point. otherwise, it gets random answers and you get lost. 
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Regards,
Karthik