How to generate topology files for a peptide having unnatural amino acid and a flexible linker

Dear All,

I want to generate (.itp) topology files for the following peptide having an unnatural amino acid and an aliphatic linker. I already tried to use CHARMM-GUI but failed.

Please suggest to me any option as pdb2gmx does not work for this.
see the attachment
Screenshot 2022-08-28 225822