You can do this by logging in as root:
docker exec -it --user=root zabbix-agent2 /bin/bash
Next, edit group file (/etc/group) in vim or sed. If You not familiar with vim, sed will be easier:
sed -i "s/docker:x:281:/docker:x:281:zabbix/g" /etc/group
Next, type "exit" to exit container shell, then You can restart container from Unraid GUI
BTW: Some trick could be used for things that not working from zabbix container, by setting up cron in User Scripts Plugin, this cron could monitor something, and write to file, which could be read by zabbix-agent, for example:
my script read parameters from ups tools, writing output to file on cache pool, zabbix-agent read this file and parse output to different items in zabbix.