Hello there ! This is an interesting question. When you have an external pullup connected to 5V with VDDIO being 3.3V it is not going to be a good thing. This is because, When the Pin is in Open Drain state, all the 5V drops across the internal MOSFET. Also the external 5V supply now will start fighting with the 3.3V via internal diode, this is bad for your chip.
There are a couple of solutions for this. The easiset solution for this is, "USE AN SIO". This is the kind of problem SIO is mean to solve. On an SIO you can have an external pull up with 5V while he VDDIO is 3.3 V. If you already dont know what an SIO is please go through the Pin component datasheet and PSoC3 datasheet. They are divided among the 4 Quadrants of pins on PSoC3 with one pair per quadrant. All SIOs belong to Port P12.
Thanks, I thought this might be the case but just wanted to confirm.