The iSeries will indeed mount a Samba share... as long as the Samba server does CIFS.
Many default Samba setups (d|w)on't do CIFS as it takes a bit of making to get it work properly and a 2.6+ Linux kernel.
There are other reasons, but those aren't important to your setup, unless you have other Microsoft products forcing certain protocols. (and/or Admins who blindly follow Microsoft's way to admin Windows Machines)