Adding constant powered outlet to existing switched circuit
It really depends on how the wiring was done.  If power goes first to the porch light then to the switch, there will be always-hot up in the light box.   

If power goes first to the switch then to the light, then it's likely the switch interrupts power to the light.  In that case you will need to bypass the switch and do one of the following:


have the porch light operate on a photocell or motion sensor
abandon the porch light
go with any of a variety of smart-switch-control technologies that lets you put a receiver in the lamp base, and a transmitter in the switch location.  For instance the X10 system goes back to the 1980s. Or newer systems allow you to control the light from New Zealand with your phone.