hi…i’m new in BTS…but i think i saw something with the multi part message that might solve your problem….
Extracting from the website http://msdn.microsoft.com/en-us/library/aa546739.aspx
From your project, add a reference to Microsoft.XLANGs.BaseTypes.
Create a variable (for example xlangPart) of type Microsoft.XLANGs.BaseTypes.XLANGMessage.
Call xlangPart.AddPart(…) using the appropriate arguments from an Expression shape.