Hi 9.G,
in my company, according to EU-OPS 1.1120, cdr discretion to extend FDP during actual ops (not in planning of course) is allowed for unforseen circumstances only, after consultation with all other crew member and limited to max 2 hrs if minimum flight crew or max 3 hrs if augmented flight crew.
Those figures have to be applied to the basic daily FDP (OPS 1.1105).