I am not sure what you’re trying to accomplish, but I wouldn’t be surprised if your understanding is incorrect.
To connect two glyphs as a ligature automatically you must use an OpenType feature called GSUB (Glyph Substitution), which FontCreator does not support. You can add GSUBs with VOLT or with OpenType Compiler.
When the user types “difficult” the OpenType GSUB feature changes that to “difficult” — is that what you want, or something similar?