With a quick web search I found
Dataset of AMBER force field parameters of drugs, natural products and steroids for simulations using GROMACS.
Jennifer Loschwitz, Anna JÃ¤ckering, Monika Keutmann, Maryam Olagunju, Olujide O.Olubiyi, Birgit Strodel
Data in Brief,Volume 35, April 2021, 106948
They used GAFF to derive models using S instead of Se, defined some modified Se parameters, and changed S to Se after all the model building, which may be an approach to use with CGenFF and CHARMM-GUI. The Se parameters in that paper may or may not be transferrable, and should be validated before use with CHARMM force fields.