You can connect a SIP ATA directly to the Toshiba if you have an IP endpoint license and an MIPU card installed in the CIX. I think most SIP ATAs would be able to connect to the Toshiba system. You would program them just like any SIP phone.
I don't know your level of experience with SIP phones or Toshiba. Sometimes SIP can be frustrating if you have not done it before, especially with Toshiba.