no the "mode" is either automatic (autoflower plants that have ruderalis in them) or photoperiod or the more normal type (kind that most people I read grow). The 12 hours of dark makes it flower (lack of light for 12 hours triggers a cascade producing flowering hormone).
Adding the T5 will do best near the bottom/darkest areas, get it close to the plants with being aware of temperature. Rotating plants around this side lighting to get an equal spread (ie 1/4 turn daily) will help.