BPM - Message merge

Hi,

I am trying merge sales orders from 2 of our systems. I get an XML idoc from my R/3 system and a partial sales order doc from my other leagacy system in XML format. I want to combine these into a single sales order and send it out in an ecxXML format to a customer. I am running WAS 6.40 and using XI 3.0. Is BPM the solution, if so How can I proceed???

Any help is appreciated.

-Kenneth