I think you’d have to manually do this through the CLI, at least right now. I’ve gathered probably needs to go into /etc/ipsec.d/certs, at least according to this documentation: ipsec.d/cacerts - strongSwan
I wonder if you could get most of the client portion setup in the controller interface then finish up with uploading the cert to the correct directory? If you’re using Windows then maybe something like https://mobaxterm.mobatek.net/ might make it easier, since it provides a little GUI to look at the file system if you SSH into the router.