You can use the Set-NaVfilerStorage cmdlet to add volumes to a vfiler.
For example, to add the volume tmpvol to the vfiler vfiler1:
PS C:\> Set-NaVfilerStorage -AddStorage /vol/tmpvol -Name vfiler1
Name Status Ipspace VfnetCount VfstoreCount AdminHost
---- ------ ------- ---------- ------------ ---------
vfiler1 running default-ipspace 1 2
PS C:\> Get-NaVfiler vfiler1 | select -ExpandProperty VfStores
IsEtc Path Status
----- ---- ------
True /vol/vfiler1 online
False /vol/tmpvol online
Hope that helps!
-Steven