Sort field in sap script (when execute F.27)

Hi, all.

I have a problem about Sort field in sap script.

Please read below and give me any advice.

[Background]

I created a custom FORM which is used by F.27 (customer statement).

It's a copy of F140_CUS_STAT_01

I chose P1(Document date, reference/doc.no.) as line item sorting in print program (RFKORD10).

[Problem]

I have a request that sorting in billing document number.(BSID-KIDNO or VBELN)

So, I tried to make sort variant in Tr-CD :O7S6.

But there are not field of billing document number.

[Question]

How can I sort line items by billing document number?

(If possible, I want to realize it by making sort variant only.)

Thanks,

Best regards.

TM