how to connect multiple proximity sensors to a PIC to work together on a single port
My answer is no you can't but there is some way to reduce the number of ports used to 4 ports, assuming one of port can be used as ADC port and the other 3 ports can be used as logic output (so it depends the PIC you use, look at the datasheet.)

Use alanog proximity sensors like this and DG528 analog multiplexer. Assuming you have heard about multiplexer, DG528 is the analog-version-multiplexer that have 8 analog input channels and 1 analog output channel. Look at the page 7 of the datasheet of DG528. Depending on the input value of address input, pin A2, A1 and A0, the output channel(pin D) is connected to one of input channel(pin S1 to S8). So you should connect the output channel to a ADC port of PIC to read, and connect A2-A0 ports to the other three digital output ports of the PIC. In this way you can read sensors with reduced number of ports of the PIC.