The problem is not having a function on the Arduino to make a custom char, it is that the PIC (with its current program) does not have a protocol for creating custom chars. As a result it would be impossible to make custom chars without re-programming the PIC.
As an alternative you might consider removing the board with the PIC on it to expose the display's original header. Then, as the display uses a Hitachi HD44780 (or compatible) controller, you should be able to set it up using the built-in LiquidCrystal Arduino library.
If you use the built-in Hitachi compatible library making custom chars is quite simple:
1: Create a byte to store your char:
byte myChar[8] = {
0b00000,
0b00000,
0b00000,
0b00000,
0b00000,
0b00000,
0b00000,
0b00000
};
2: In your init phase initialize the char:
lcd.createChar(0, myChar); // lcd.createChar(int, byte)
3: Finally you can print/write the char using its identifier (the int you passed in lcd.createChar):
lcd.write(byte(0));