Curvy and straight paths

Hi,

I'd like to create a shape that has some straight sides and some curvy
(B-spline or bezier) sides. I vaguely recall in the distant past being
able to toggle vertices between corners and curves but I can't seem to
find that option.

I also had a read of the manual and it also mentions being able to
join or compose paths, but it says those are in the Edit menu and as
far as I can see they aren't.

Is there any way to do this?

Cheers,

Tim