Hi @b-omics
I am not super familiar with parametrization of proteins, but for sure if you want to use CHARMM36m then a very good starting point is the CHARMM-GUI website where you can load your pdb and have the webserver parametrize it for you.
I never did it for special aminoacids so I do not know if you can find what you need, but there are also a few tutorials on the website itself that will help you. You can also do it with gmx pdb2gmx if you have the CHARMM36m new forcefield distro. That might be more complex if you are new to GROMACS way of formatting files, but there are also old posts that can address problems that you may face in this case.