The following discusses how to add a disk to an existing system:

http://netbsd.org/docs/misc/index.html#adding-a-disk

However, this document only mentions the addition of a single slice.
This generates three questions:

0.  disklabel asks for the starting offset in:  [0c, 0s, 0M].  Is the
expected response only in cylinders, or sectors, or megabytes, or do
all three values need to be specified?
1.  Is the same true for the partition size?
2.  If a swap partition is specified on a new disk, what needs to be
done to notify the system that it is to split the existing swap across
two disks?  ie.  I created two partitions on a new disk, swap &
4.2BSD.  After mounting the 4.2BSD partition, I saw no change in the
amount of swap reported by top(1).

Thanks for any candor shared.

Jim