I don't think it is possible to add a symbol to the chart. That is hard coded inside Pro/E and is backed by a file called special.fnt found (on windows systems) in <loadpoint>\i486nt\text\usascii. That is a compiled file, the source code for which is found in <loadpoint>\text\usascii\special.src.
While you can modify the special.src file and re-compile it to make your own symbols, the symbols shown on the button faces will not change. Or at least I've not been able to get them to.
The compiler is found in <loadpoint>\i486nt\obj\compile_font.exe
Unfortunately all the instructions for modifying the source file have ceased to be printed in any books or help for any version since version 18.