Pretty much as Nazul says:
For step-by-step:
Create a 'new' channel under the channels tab.
Copy/paste the black + white Bryce mask render into that new channel spot.
Then you can use it as a selection however you like.

RosemaryR
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"This...this is magnificent!"
"Oh, yeah. Ooooo. Aaaaah. That's how it starts.
Then, later, there's ...running. And....screaming."