I think this is possible, but you will have to do some config to get the incoming address of your "special" non-scalix to be "stamped" with mailnodes that you can then use to control access.
Typically when an external mail arrives onto a scalix server the address that Scalix passes around the system is something like:
S=JG/OU1=internet/INTERNET-ADDR=jg@foobar.com
OU1=internet is the mailnode of the default internet mail gateway, see:
omshowrt -q unix
So, what you could do is add a specific directory entry for your external user, e.g.
G=Special/S=User/CN=Display Name/OU1=InternetAllow/INTERNET-ADDR=jg@foobar.com
when
jg@foobar.com sends mail to the Scalix server the From: address will be specificed as above.
Now you have a way to differentiate this "external" user from others. You can now add specific ACIs for this mailnode (i.e. OU1=InternetAllow), which should then enable this external user to use the PDL.
One last step you will need is to add a route for the above mailnode to ensure that any replies from Scalix users are sent back to the internet, i.e.
omaddrt -m "InternetAllow" -q unix
If you have have multiple users on the external mailserver that you wish to allow then things become a little more complicated. You could add directory entries for each of these users but this maybe problematic. There are some "old" configuration files that can be used to map internet domains to mailnodes, but you are getting into deep water here and this type of configuration hasn't been tested for a while.
Hope that gives you some ideas to play with.
Cheers,
JG