Well easily no, but yes I've done it. One client insistent on being able to do this and we went through the initial idea creating user records on the fly in the process and then using that as the assigned user to send the message. This worked fine, but in the end they didn't want such records being created. It gave the potential for these users to try and access self service and there were a lot of them!
So I replaced the whole of the LANDESK reminder feature with a business object plus calculations throwing data out via event manager to another app which just sent the emails. It has been very solid ever since I put it in, but of course it doesn't have the same retry features as LD has.
I seem to recall that in a recent-ish release reminders have an action which allowed an external address to be specified, but that action wasn't accessible in a process, only manually (Duhh!) If that has changed, then it might be worth looking into
I would use the cc recipient functionality with process decisions checking the value of the attribute. The only downside of that of course is every time the originator is notified, so is the cc recipient.
I was able to accomplish this using CC recipients. Thanks!