Yes, you can add new domains to Cloudron. Depending on where the authoritative servers are for the domain, you can use the dedicated API-based setups (i.e. AWS, DO, Namecheap, Linode, etc), or you can instead use the Wildcard setup for any other service that's not explicitly listed in the providers list.
I have a non-Cloudron site installed at social.mydomain.com. Sorry, that part of my post may have been confusing or ambiguous.
Personally I simply go directly at your DNS manager (Namecheap, Cloudflare, GoDaddy, ...) than add the entry manually, such as you would do without Cloudron.
It also confused me a few weeks ago but at the end Cloudron only add and manage entries it need such as:
A for my.
MX for mail (if you active them and decide to receive email)
TTX for DMARC, DKIM and SPF
in your case you will have to add something like
Entry Type: A
Content: TheIPofYour "social.mydomain.com"
since we are all graphic bug, here a sample of my zone at Cloudron
the one in red are the one I added manually.