FontCreator 8 allows you to edit OpenType layout glyph positioning (GPOS) including, but not limited to pair adjustment (also know as kerning pairs).
However over the years we’ve spent a lot of time improving this part of FontCreator, so we strongly recommend to upgrade to the latest version.
If this is a symbol font, it is recommended to convert it to Unicode from the Tools menu. Then you won’t be limited to just 256 characters.
It seems that what you want is a standard ligature feature, which is usually what we use for Alphabetic Presentation forms like ff, fi, ffi, fl, ffl, etc.