Symbols, symbols... Well, all, that I could find - were using "Resourse hacker" to extrct and then replace the "ExFont" file inside "RPG_RT.exe" file. I've never did this before, and can't say for sure, but you can try. The infos says that you need hard black-white (Not even shades of gray) 156x22 pixel sized picture (Every symbol is 12x12). Then just find (In the resourse hacker) Bitmap -> EXFONT and there should it be. To extract choose Action -> Save and choose where. To replace choose Action ->Replace bitmap ->Open file with new bitmap. You can re-read guide that I used at https://rpgmaker.net/tutorials/1400/ it have screenshots.
I hope that will help.