No, you need to use a headset with a "Partner Base", which is really a single-line non-dial telephone with a headset that plugs into the AUX jack on the Partner phone. Then you turn it on and dial from the keypad of the Partner phone.
Or, there is a left-handed (no offense!) way to do it, but it leaves the extension in an off hook state when you are using the headset but not active on a call. Assign a phantom line to the extension in question, use something like line 15 or so, something that doesn't really exsist on the system. Connect the headset normally, using the handset jack, and leave the handset out of the cradle to keep the hookswitch up. When you are NOT on a call, press the phantom line key. You will instead be off hook on the phantom line. Then press the line or intercom you want to use, and go back to the phantom when you're done. Put the handset back in the cradle when you're done using the headset.