Hey,
I don't think there is a way to add relay domains via DA but... (You might be able to just add them to the /etc/virtual/domains file but, that might be overwritten.
I think you might be able to do something like this:
Create a files in /etc/virtual called "relay_domains"
Put the domains you want to relay for in there. One domain per line (like the "domains" file in that same directory).
Next, change the line in the exim.conf file that looks like this:
domainlist relay_domains = lsearch;/etc/virtual/domains : localhost
and make it look like this:
domainlist relay_domains = lsearch;/etc/virtual/domains : localhost : /etc/virtual/relay_domains
Set the "relay_domains" file to be mail:mail and 644 for permissions (I'm guessing here).
Now, I haven't tested any of this, so use at your own risk.
Be sure to backup the exim.conf before you edit it!
Hope that is at least going in the right direction...
David