It IS possible-I exclusively expressed for a year, but it does take a lot of dedication. I've c+p what I once fair about it elsewhere
Admittedly it was really hard for the first few weeks, as the way I thought of it was that I had to be a feed "ahead", so it meant that I had to have a bottle of milk ready to feed before I expressed the next bottle. I did it by religiously expressing every 2 hours or so (mind you at times the little so and so was feeding every 2 hours!), so that if he was actually feeding every 3 hours, I was getting a little stock built up. (I'm sorry if this reads like gobbledegook but it's sooo hard to get across in type rather than speech)
After a week or so of expressing every couple of hours, I found that I then had maybe 2 or 3 bottles of milk in the fridge, so that's when I moved on to expressing after I'd fed him, so I was effectively replacing what he'd just had from the fridge.
The 1 thing I had to be careful about was if I was out and about and he was having a bottle of milk, then I wasn't getting the chance to express until I got home, so you have to watch you don't leave it too long between each expressing session in the early days. Then again, if I did leave it a bit too long, I felt like my boobs were going to explode anyway!
I'm not sure if this is your 1st baby, so I'm saying this on the assumption that it is! After a couple of months of feeding, your breasts almost produce milk on demand so you don't get the "full" feeling that you do in the early days, and I was able to relax a bit more about when I expressed, but still working on the idea that if he had bottles in a day, then I expressed 4 times a day.
I never set out to breast feed for as long as I did, but that's how well the expressing worked for me, so I didn't see the point in spending money on formula if I could do it myself!
What I would say though, from speaking with other mums, is that I seem to be able to express a lot of milk very easily, and not everyone is able to do it as easily. Maybe that was the case, or maybe it was because I was so "religious" about it in the early days-who knows?
I probably come over all evangelical about it, but it's just what worked for me, and it's not something that's usually presented as an option at ante-natal classes/baby clinics etc. No matter how successful I was at breast feeding, or expressing, I'm still very much of the opinion that each mum has to do what's best for her and her baby....and for some people that might mean either using formula or a combination of breast and formula. There are a lot of "breast feeding !!!!!" around, especially if it's your first baby, but IGNORE THEM and do what's best for you and your baby.
I hope I've explained it in a remotely understandable way, but if there's any part which doesn't make sense, please get back to me and I'll try to explain it a bit better.
Oh, and if you do manage to do something like what I did, remember, and make sure that those around you remember that you're breast feeding even if it's coming via a bottle, and you do need to have allowances made for the extra load it places on your body (god I sound so old fashioned!)