According to the manual page of the vnconfig command, it should
   allow you to start swapping on a file, by hooking this file up to
   a special vn-device. I haven't actually tried this (has someone?),
   but it looks like an option for additional swapspace at runtime.

I would also like to be able to add partitions with swapon at runtime
when I have not configured them into the kernel.  This hasn't worked
for me, and I don't see a reason why it couldn't.  

Removing partitions at runtime is obviously a stickier issue.  As I
understand it, netbsd sprinkles pages evenly across all the active
swap partitions.  This makes it difficult to remove partitions; the
kernel would need some special swap-reassigning code.  I presume that
to allow removal of active swap areas, Linux just allocates linearly
so that later space you haven't consumed yet is always removable.

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