Once you are to the point of having a vinum partition on the new
disk, create a file with the commands to add the drive and the
subdisk. For example, if your current /etc/vinum.conf looks as such:

drive wd200GB device /dev/ad0s1d
volume span
plex name span.p0 org concat
sd name span.p0.s0 length 190779m drive wd200GB

And you want to another drive, ad3s1d into the span volume, create a file with the following:

drive wd1200JB device /dev/ad3s1d
subdisk length 0 plex span.p0 drive wd1200JB

Initally, the plex will show a state of 'Corrupt' and the new subdisk will show 'Empty', when you run vinum list

Stop the plex vinum stop span.p0, and it will automatically, move to an up state, likewise for the subdisk