The Explorer pHAT is sort of an experimental board, so I always advise to use a stacking header with it, which allow you to access any GPIO, as seen here:
If you have already soldered the header, then you should still be able to solder a wire or component directly to the pins you require by remelting the relevant joint.
When it comes to the transducer I'm not sure, but I think it's a matter of choosing your current-limiting resistor according to the nature of the supply... I can't see why you'd be required to use 3v3.