How to Hold Sender Filename for Receiver Filename

Question

Hello,
I have a File (xml) -> PI -> File (flat) scenario where I need to maintain a part of the sender filename and use it in the receiver filename.  This value can be one of three values (if that matters). 
I envision having to get the filename value into my payload and then using receiver filename substitution.  But how would I get the filename into my payload?  Is there another way to "carry" part of the filename over to my output?
example:
sender files:  A.123.X, A.456.X, A.789.X
- need to do mapping into new format and create...
receiver files: B.123, B.456, B.789
(the 123, 456 and 789 are not in the original payload data)
Thanks,
Keith

Answer 1

More details on my issue...
When I remove my XSLT mapping to see if my first message mapping with UDF to get the filename is working....it is.
<?xml version="1.0" encoding="UTF-8" ?>
<ns0:UPS_EBR_BILL xmlns:ns0="http://www.ups.com/Billing/v1">
  <ns0:InvoiceDetails>
    <ns0:Invoice>
      <ns0:InvoiceNumber>000000082648018</ns0:InvoiceNumber>
      <ns0:File>APGI-20080105-xml.zip</ns0:File>
Then the aforementioned ns0 error happens when my XSLT map runs.  Here is the top of my XSLT.  Any suggestions on how to adjust the XSLT to handle the new ns0 or any way to get rid of the ns0 before XSLT mapping?
<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:ns0="http://www.ups.com/XMLSchema/EBR/Billing/v1" xmlns:ns1="http://graybar.com/cold/invoice/">
   <xsl:output method="xml"/>
   <xsl:template match="/">
      <ns1:ColdInvoiceData xmlns:ns1="http://graybar.com/cold/invoice/">
         <xsl:for-each select="ns0:UPS_EBR_BILL/ns0:InvoiceDetails/ns0:Invoice">
                    <Invoice>