In order to pressurize the flight deck, one would require a higher source pressure just for the flight deck, and this doesn't exist, nor does any way to provide it exist, in the pneumatic system, after the air conditioning packs are done preparing the air for cabin use.
Guppy, it has been suggested in a previous message thread on a similar topic that, on 744 Freighters with Main Deck Fire Suppression systems, that a slightly positive pressure is maintained on the upper deck when the lower deck is depressurised to 25,000' (when the Main Deck Fire Suppression system is activated).
Airconditioning to the Main Deck is cut off and Pack/s in low flow mode provide air to the cockpit.