You can make a bi directional level shifter for the I2C signals and feed your digital pot with 5V.
For the I2C line data line:-
Take two NPN transistors and connect the bases together. Connect the emitter of one to the collector of the other, and the collector of one to the emitter of the other. Connect the bases through a 1K resistor to 3V3.
Then one collector / emitter goes to the 3V3 signal and the other to the 5V signal.
You can repeat this for the clock line or have a two stage transistor common emitter amplifier. You need two of these so that the clock signal is the correct way up.