FFTacText can edit most of the text that appears in game. It is included in the FFTPatcher Suite
There are several caveats, though:
1) Each file has its own maximum upper limit in file size. You should not add too much text (though some space can be saved by deleting "no messages" on spell quotes).
2) Don't extract text from an extended ISO. Even if the text is fine, some of the non-text data may not be, and there is a possibility you cannot patch your TacText file. If you did this, always use the restore button with an unmodified ISO on files with malfunctioning DTE and manually redo your changes.