Kavita,
XIF Adpater is directly linked to BDoc Adaper as well for processing. Also, the standard best practice to extend XIF IDOCs is using transaction BDFG as it involves both BDoc adpater, AET & EEWB changes.
I would not suggest directly updating the XIF IDOC segment as the set up doesn't permit that.
You can always create a Custom IDOC for the requirement but then you have to trigger the IDOC programmatically.
Rgds,
Ravi