I have a 220vac 3 wire extension cord to which I wish to add a 110vac outlet as a convenience. Can this be done safely since there is no ground wire?

If this was #10 or larger, you could probably use a 4 circuit panel, 1 240 volt circuit and 1 or 2 120 volt circuits and have your 120 and 240 volt recepticles mounted through box.

Edit, I noticed you said 3 wires, Black, white, and green?

I'd say no. We need to know the application. You can buy portable cordage by the foot in 4 conductor.