Your first question was if the mail to admin and user was setup by default. So that is not so hard to imagine for DA.

The "and external email address" was an additional question.
I only answered the first question if it was enabled by default or not.
As for the second question, does it need to be to user+admin+external address? Or only user+external address?
Because if you want your admin email to be an external email address, you can easily change your admin email address.
If you want to keep your current admin email address you can also setup a forward for your admin email to an external email address. In that case welcome emails are send both to the admin email address and the forwarder.
However in that case all mail send to admin email are also send to that external address too.
There might be an option to change it in the welcome email templates but I'm not sure about that, you can have a look.
/usr/local/directadmin/data/admin/a_welcome.txt - used when adding new admins
/usr/local/directadmin/data/admin/r_welcome.txt - used when adding new resellers
/usr/local/directadmin/data/users/<resellername>/u_welcome.txt - used when adding new users for this reseller
Hope that helps you.