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 Jäckering, 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.

Rick Venable
computational chemist