Subroutines in CalculiX not only may have a different syntax but they require recompiling CalculiX from the source code. But you can find some ready-made examples and workarounds. This thread might be helpful: UMAT option in PrePoMax
PrePoMax is not a problem here as you will only need to configure CalculiX properly and add necessary keywords - you can just use the keyword editor for that.
Also, it might be good to ask on the CalculiX forum. Someone there should have more experience with CalculiX subroutines.