it could be done, but it would be easier to just get a bigger vessel. after so many steps, it might stop growing yeast, since you'll have so much in there. you could pour off most of it into another jar and keep doing steps in the flask though.
It can be done. Make a 1.5 liter starter, when it is finished pour all but 250ml into a cleaned and sanitized vessel. Don't crash it in this case as you want to keep the yeast in suspension. Add another 1.25 liters of fresh wort and repeat. It may take some time to do this but it is possible.
I think you would have to use lower pitch rates than most homebrewers would recommend. If you can build up 400-500 billion cells it can work.
Last Sunday I brewed 5.2 gallons of 1.053 CAP. When it had cooled to 60F in the kettle and I had aerated it best I could I took just under 2 quarts of the wort into a growler and pithched my swollen pack of 2035. I put the rest of the wort in the fermenter and in to my chest freezer that was set to 40F or so. I did not even bother shaking the starter. It was kept at around 60F on the floor in my house.The next morning I pitched the starter into 50F wort.
Yesterday (Sunday again) I pulled the fermenter out and let it warm up to room temp. Today I took a gravity sample. 1.010 and tastes good. I'll crash it soon and use the yeast for a 10 gallon batch this weekend. I'll be using close to 20 million cells per ml then.
I figure I pitched it at about 9-10 million cells per ml. With a healthy culture that number can work quite well.