Seems to be very difficult... As a workaround I use a little script to enable the Enpass repository before checking for updates and disable it afterwards. This way I don't get error messages all the day about the repository not being accessible.
The script is for Tumbleweed:
#!/bin/bash
# Enable Enpass repository
printf "\n -------------------------------------------------------------------\n"
printf " Enabling Enpass repository\n"
printf " -------------------------------------------------------------------\n\n"
zypper mr -e Enpass
[...]
zypper refresh
[...]
# Dist-upgrade with no-vendor-change
zypper dup --no-allow-vendor-change
# Disable Enpass repository
printf "\n -------------------------------------------------------------------\n"
printf " Disabling Enpass repository\n"
printf " -------------------------------------------------------------------\n\n"
zypper mr -d Enpass