If you have messages in SMAT which represent the messages you want to test with then yes just extract using the SMAT tool to either a new line terminated or length encoded file.
If there are many messages I want to test with I put each message in a file – I find that easier to work with.
If there are no existing messages maybe because this is a new source system and there are no messages to be had, you can actually use the Xlate and the Xlate Testing tool to craft a test message according to the Source System’s spec. Save the message in the Testing tool to a file then use that file to test the new Xlate.
This is a technique I cover in my Xlate class.
email: jim.kosloskey@jim-kosloskey.com 29+ years Cloverleaf, 59 years IT - old fart.