No, a Pope could not consecrate a bishop "in pectore". Sacraments cannot be conferred that way.
He could in theory "appoint" a bishop to a diocese or other position, or could "designate" that someone should become a bishop, but as per your link, these cardinals "in pectore" acquire no rights until their appointments are made public, and so a bishop "in pectore" would be meaningless.
Now, there have been "secret" consecrations, where the Church thought it best not to announce that an inividual was had been consecrated a bishop because he might be, for example, operating in an area (behind the old Iron Curtain) where he would be arrested, perhaps killed, and unable to carry out his mission.