Hello Jan-Piet.
Is it currently possible with Knot to sign a slaved zone?
Unfortunately, this is not possible.
Current Knot DNS internals don't play well with multiple sources of
changes to the zone data. This is the reason why we decided to turn off
incoming XFRs for automatically signed zones.
You aren't the first person asking this quetion. And having Knot DNS as
a bump-in-the-wire solution for DNSSEC is actually a nice feature. I
think we will implement it at some point. But I can't promise when.
Best regards.
Jan