This is needed for a place where there's a high pressure water but not always available.
Yes, no reason this can't work, within the limits of the pressure tank (alternate name for "bladder accumulator" not that there is anything incorrect about that term - suspect it's regional differences in terms.) The one on my well pump system stores ~30 liters of water, but they come in many sizes and can be combined, so you should be able to store as much as you like within the limit of providing space for them.
Certainly far more practical than trying to create a piston to do the same job, per your other question. You will need to be sure that your accumulator is safe for use at the "higher than average pressure" of your building, or that you use a pressure regulator to reduce the pressure to the maximum that is practical for the accumulator; and you will need a check valve (one-way valve) on the input so that your accumulator does not send water back into the building when the water pressure falls.
You may also need to provide yourself with a pair of pressure gauges - one on the incoming line before the check valve, and one on your side of the check valve, or you won't know when the building pressure has fallen and you are using the limited supply of water you have stored.
There is no need to elevate the accumulator - internal air pressure on the bladder full of water or diaphragm separating water and air provides the pressure, not elevation of the tank.
Responding to comments The type of pressure tank I am most familiar (in the modern era - I do have experience with the old air over water tanks, and I hate them) has either a bag/bladder or diaphragm which separates a captive air charge from the water. The captive air charge is pre-pressurized to a few psi below the lowest expected working pressure (so on my well pump, the tank is pressurized to 38PSI, the pump starts at 40 PSI, and the pump stops at 60 PSI.) At 2.7 feet per PSI, moving the tank up 2 meters/6+ feet makes barely over 2 PSI difference - the last bit of water to come out of the tank (if the pump is shut off and does not start) comes out at 38PSI (due to the pre-charge on the tank) so there's no practical difference in moving it higher.
In setting a tank used the way you describe, the air charge pre-pressuerization will need to be set so that the maximum pressure does not exceed the flexibility of the diaphragm/bladder and burst it. Whatever that setting needs to be will define the lowest pressure you will get from the system just as the last bit of water runs out of it. The input pressure will determine the highest pressure you can expect (unless it's too high for the tank/accumulator at all, in which case you will need a pressure regulator to lower it, and THAT will set the lowest pressure you can expect.) There is a trade-off - the higher the pressure, the less water your tank will store - the lowest values typical in household water system here are "On at 20, off at 40" (Pounds per square inch - roughly 1.4bar to 2.7 bar) and for any given tank, that will store the most water. "On at 30, off at 50" and "on at 40, off at 60" are other common values for cycling well-pump type systems. Municipal systems are, I think, commonly regulated to below 100 or perhaps 80 PSI if the system pressure is higher than that at a given house.
Other than price, it makes no difference if you have 400 tiny pressure tanks or one big one, if the specifications are otherwise the same. I shopped on both physical size (what would fit into my space) and price per capacity of tank (where there was definitely a minimum at a certain size, so two of that size where cheaper to purchase than one twice as large - but the larger one would be more space efficient and require less plumbing.)
Normal domestic water diaphragm accumulators (usually bladder accumulators are used) would not be feasible as they are used with a pump that cuts off when max tank pressure is reached. When you say "use them as a water source" please give an idea of estimated capacity needed at what pressure. Perhaps you should be using a simple raised atmospheric accumulator (water tower) that relies on gravity to supply pressure when your "high pressure water" is not available. 8 to 10 meters height should give enough usable pressure, higher = more pressure.