It seems like it should be possible. Celexa is an SSRI, meaning it increases the levels of the hormone serotonin floating around in your body. One of the things serotonin does is decrease appetite, so maintaining the psychological aspect of fat loss would be easier.
One thing to keep in mind is serotonin levels are very sensitive to the amino acid profile of your food. Serotonin is made from the amino acid tryptophan, which competes with other aminos for absorption. Fruits and vegetables increase serotonin levels (good) while whole grains inhibit serotonin production (not good). I would venture a guess that dairy also inhibits serotonin since it is very high in the amino acid leucine, which directly competes with tryptophan.
Another thing to keep in mind is that muscle proteins use every amino acid except for tryptophan to rebuild. So having more muscle will increase the ratio of tryptophan to other amino acids in your body by "using up" the other aminos as long as you engage in some kind of strength training. That means more serotonin for you.
So, sorry for the biology lesson, but I guess the takeaways are:
1) fat loss on Celexa is possible, when
2) you emphasize fruits and vegetables over grains and dairy, and
3) build your muscles through strength training activity.
1) eat real food - more vegetables, moderate meat, moderate fruits, less grains, less sugar, less vegetable oils.
2) exercise - moderate intensity cardio, sprinting, heavy lifting, dedicated stretching and mobility.
3) live - relax, de-stress, meditate.
Disclaimer: I'm not professionally qualified to make any formal recommendations. I've just done my homework and I'm my own guinea pig. All of my data, unless otherwise cited, comes from a sample size of n=1 (me).