I found the program by searching Google for sflib10c.zip, and have it installed in a Windows 98 VM. I don't know if it works on anything more modern than that. But anyway here's the basic gist of it once you have it installed:

1. Run the program (by default in W98: C:\Program Files\E-mu Systems\SoundFont Librarian\SFL32.exe)
2. Once running, open up the .sbk file (File -> Open).
3. A window pops up asking you to convert it to .sf2, click yes and save.
4. Open the newly-converted .sf2. A sub-window appears with the .sf2 loaded.
5. From the top drop-down menu, click on Edit -> ROM to RAM Samples.
6. Ctrl-S or File -> Save to save the sf2.

Soundfont Librarian was a tool released by E-mu some time in the late 90's somewhere online. It wasn't really a full-scale soundfont editor, per se, but it had at least basic editing and management functionality for both .sbk and .sf2 soundfonts, including the ability to convert the former to the latter.

This program is actually where the heavily-circulated 1mgm.sf2 (official AWE32/64 'ROM Dump' soundfont) originated from. In fact, it seems that this soundfont originally existed to aid in the process of converting old AWE-era .sbks to the newer .sf2 format, instead of it being meant for general use.

Just wanted to spread the word for anyone who really cares about this sort of stuff. I only wanted to find a better way to convert synthgs.sbk and synthmt.sbk in a way that didn't require lots of manually replacing ROM samples in Viena + Polyphone.