I suppose you can't add any symbol to the sytem SYmbol pallete.However you can you have your own symbol pallete(aka Symbol Instance Pallette).This is nothing but a valid Pro/E drawing in which are inserted the desired symbols.
To elaborate the procedure...
1.First you create the symbols you want to be included in the proposed Symbol Instance Pallette.
2.Then create a new drawing file with a name something like user_sym_pallette.
3.Insert the symbols you desire to be appeared in the Symbol instance palette,in to this drawing at apporpriate places.
3.Set the config.pro option SYMBOL_INSTANCE_PALETTE_FILE with value as the absolute path to the drawing just created.
4.From now on when you are about to insert symbol in to any drawing click on PICK IN PALETTE button from the Symbol Instance dailog to get the Symbol Instance Palette and pick the symbol from it.