Non-standard compiler integration

I try to build a project with cmake, that uses a quite special compiler. It is about the xcc compiler from MicroSys for the os OS-9.
The interface of the compiler differs from other compilers, like its objects files are instead of .o files .r files. All argument specifiers are different (eg. -L is -l) and so on.
I was not able to find a documentation on how to add a custom compiler to cmake. Can anybody help, or knows where to find the documentation?

Best regards

Looking at their homepage I did not found xcc, but their OS-9 compiler seems to be Clang (based).
So it may work pretty much already. Or is it an older self-made compiler?

Maybe you need to write a platform file, if the file extension is specific for OS-9 itself and not the compiler.

Writing compiler or platform files is not really documented. But there are enough other files to copy&paste from.