You don't have to use the bpm:assignee aspect. It's just a short-cut for adding the appropriate meta-data to your task for collecting an assignee. It just so happens to name the assignee slot bpm:assignee and it's this name that's referred to throughout the out-of-the-box workflows.
Take a look inside the aspect, and you'll see it's defining an association. That's the important part. You can either create your own aspect via copy/paste (and rename both aspect and assoc), or just add an equivalent assocation direct to your task type (renamed too). In your association, remove the mandatory constraint.
Then in your process definition, refer to your name, not bpm:assignee.