It is not easy, even with right tools.

I still dont understand for what you need extra M-function.
Maybe if you explain in detail what you want to do with this M-function, somebody will help you.

Every action in the machine has its own Marker in the PLC.
90% of the job is finding the right markers, especially if you are doing the m-function by reverse engineering the manufacturers PLC.

Heidenhain general purpose markers are easy to find.
Machine manufacturer are the troubling ones.

Also,
You need set of following heidenhain programs:
tncremo - easy to acquire
PLCdesign - hard to acquire
PLCtext - hard to acquire
Cycledesign - hard to acquire

And dont send me PMs, I rarely have time to be on this forum, maybe once for 3 months, anyway I wish you good luck with this project.