That is on the wishlist, likely won't be high enough to be in the next release though.
You can currently edit the SOA with this guide: http://help.directadmin.com/item.php?id=87
If the change you're doing is permanent, then copy the named.db to your custom directory (it's within the templates directory)
You can use this method for just certain domains to be changed, even though they all use the template. The if-then-else functions can be used in all templates, so you can work it to have certain SOAs for some domains, and different onces for others. http://www.directadmin.com/features.php?id=430