Well, if you're talking about FF7 then that's *exactly* what my editor, Cosmo, is designed for
For FF8 you have Garden, written by Qhimm.
However, neither is 'complete'. Cosmo still isn't 100% compatible; some levels it'll fail to edit, or break so they won't load properly. Nevertheless, you can edit quite a lot of the data with it.
Garden, I can't really comment on, but it's a definite beta app.
You can get Cosmo from my site
http://ficedula.cjb.net - a bit out of date, but I haven't updated Cosmo in the past few months anyway, so that should be alright...