Its not really possible to change the External channels into a PH probe input. The main reasons for this are that the PH Probe is bi-polar (approx. ±576mV) and the Ext inputs are unipolar (0 to 2.5V). Also, the PH input is heavily filtered, so you would need to apply this filtering yourself.
I would not take this on unless you are reasonably confident with electronic circuit design.
The current through the Digital Out is supplied by the PC, and it is usually very low. You could use an external power supply and an amplifier, but again, you would need to be reasonably confident in electronics (sadly we don't have any circuit designs for this here).
Sorry I can't be more optimistic with this