I don't know how to modify the default palette. You do have a definable symbol drawing which can be placed in the directory of your symbols. The drawing is a Pro/E drawing with symbols in it. Pro/E recognizes the symbol when you choose it while inserting a symbol. There is an option "Pick in Palette" available when creating a symbol. This palette is the Pro/E drawing you have defined.
The above is an example line in the config.pro telling Pro/E where to find the symbol palette.