When the source for a DMCL is not in the dictionary, is there a way to decompile the load module and produce the source that matches it?
There's no utility or method to 'decompile' a DMCL load module and get the source syntax. However, there are two ways you can get info about what is in a DMCL load module, that you could use to create compatible source.
1- You can punch the DMCL load module in hex, and use the DSects to interpret what's there;
2- You can issue a LOOK DMCL, or run IDMSLOOK in batch against the DMCL. The LOOK utility will read the load module and give you a report of what it contains. You can then create source based on that information.
Most clients find that the easiest approach is to recreate source based on the info from the LOOK output (option #2 above). You can then modify it as necessary to support your expansion.
IDMS manuals containing syntax for the Utilities can be found here: https://support.ca.com/cadocs/7/CA%20IDMS%2019%200-ENU/Bookshelf.html.