I'm not sure what the deal is with fonts, as I haven't had much interest in that problem, but I got the impression that adding fonts requires reflashing the reader with a new filesystem image, not just copying files .ttf files to the reader. But if copying worked, then yes, you've pretty much got it; you would use a command like:
prsutil put c:\tt0003m_.ttf /opt/sony/ebook/FONT/tt0003m_.ttf
Note that you need to specify the full filename in the target path, not just the directory.