Yes its very posible and not too difficult if you already have good AS knowledge but to be honest its a lot of work over nothing, online flash games really end up sucking if theres not enough people playing them and even then people dont really buy into it most of the time, when you can create a good... 'offline' game then go for multiplayer in it, otherwise your going to have to get ready for a 80% chance of flop.
Thing is most flash gamers just want a quick game , they dont want to get into it with a X360 controller and headset with HD visuals and online play.
Get a great original concept , otherwise dont waste your time.
My five dollars.