I developed a game with Cocos2D a while back, and used Glyph Designer
Is it possible to use Glyph Desiger with a non-cocos2d project, or is there any other framework you can use to make custom bitmap fonts?
Yes. Glyph Designer can also create fonts usable by Sparrow Framework and by the projects in the Learning iOS Game Programming book. Generally speaking you can write a custom loader for practically any game engine since the .fnt format is rather self-explanatory. It would be helpful though if that engine already had support for texture atlases and bitmap font rendering.
There are other bitmap font tools (Hiero, BMFont), but they're less than ideal for Mac/iOS developers. A viable alternative might be bmGlyph as it is a native Mac app.