Extract SA from J1939 Message?

I am guessing you got this one answered already, but for anyone else wondering:

The SA on a JFIN or DM1 channel is taken from the J1939 module the channel is assigned to in the system layout. 

The SA for the request PGN requesting a JFOUT with send method on request is taken from the module the JFOUT is assigned to.

In case one needs to listen to the same PGN from multiple sources, the solution is to add additional J1939 modules in the system layout and duplicate the channels. 

A related hint: If you want to use the SA constant somewhere else in the application, you could use an integer parameter in the Initialization group as the J1939 module source address.