In general, from a strictly technical perspective, the answer would involve making outlines of the font in Illustrator or another vector program, putting those into a new font file (I use FontForge to make fonts), and then adding the characters you need.
In your case, perhaps you could find some way to make Trajan work with Sell Your Soul, since the font creator seemed to base his work heavily on that. If your client's site headers are images instead of text, you could possibly use both fonts in tandem to do what you need. Maybe you could even study how the Trajan glyphs were modified for Sell Your Soul and then emulate it.
The best course of action is to contact the font's creator and see if he's willing to share his source file or give you permission to make a derivative work. Not sure if it's possible with your particular case, but you might try mining the Internet Archive's listing for his old website (Go to archive.org and search for http://www.geocities.com/crizcrack666) to see if you can get in touch with him.
Of course, doing so will possibly leave you open to some kind of legal trouble. I am not a lawyer so don't take this as real legal advice, and this stuff all depends on what country you're in. However, the font is licensed for personal use, so using it in a commercial setting as you are doesn't put you on good footing. There's no license for derivative works, so there's that. On the other hand, font licensing and copyright follow the actual font file (the "software" bit of it) rather than the actual glyphs themselves, but I could see a world where the author of either Trajan or Sell Your Soul objects to your use of it.
Sorry that's not a straight answer, but the legal system does have a way of making things murky sometimes. Regardless, it's always best to get a font with a good license and a full set of characters when you're doing professional work.