You can use ACPYPE (https://pypi.org/project/acpype/) to create topology/parameter files from Amber and/or convert them to other formats. You can create new topology/parameter/structure files with GAFF parameters in AMBER/CNS/CHARMM/GROMACS formats using a pdb/mdl/mol2 file of your residue, or you can convert an existing set of parameters from Amber to CHARMM, gmx etc format. In the latter case, you need to have a prmtop/inpcrd set for your residue (which you could create yourself with antechamber and leap first).

Also, I think that CHARMM-GUI itself supports an interface with Antechamber. I have never used it myself and therefore cannot tell you how it works and if it's suitable for your case, but it's worth checking out.

However, wouldn't it be better (i.e. more sound) if you generated CHARMM parameters for your CHARMM simulation, instead of using Amber parameters that may not be fully consistent with a CHARMM system? Unless, of course, you're going for the "create your system in CHARMM-GUI and then generate an Amber topology for it, because we use the same naming scheme" trick that the Amber people suggest in their membrane protein tutorial, in which case, try acpype and see what happens.

You follow this tutorial, http://ambermd.org/tutorials/basic/tutorial5/

After creating the topology and coordinate files, you can use ParmEd (http://parmed.github.io/ParmEd) to convert the topology file.

You can use ATB server https://atb.uq.edu.au to paramaterize some (NOT ALL) non-standart molecules.