USG Flex 500 SSL Certificate

Is it possible to add a trusted SSL Certificate to the USG Flex 500 through Nebula?  We need to use the 2FA portal for remote client VPN, but a number of users get hung up when they encounter the invalid certificate warnings on the 2FA page.
The Flex 500 uses a self-signed certificate.  While we can install the certificate on our machines, that won't help with outside contractors and vendors.
Edit: I realize this is in the "Security" forum, not Nebula - apparently I was reading Security posts prior to this.