It is always better to have equipment that is more than able to do the job.
I have a paasch single action air brush and I find that it works at it's best at around the 30 - 35 psi mark. Others may find a different pressure works best for them.
I fiddle a little with the pressure to get the finish that I want.
I also have a regulator and a water trap in the line. In fact there is a regulator on the compressor, one in the line and one on the water trap.
Trying to add an accumulator tank to your current unit might become dangerous and will not solve your problem. If the compressor can not punch more than 20psi, then adding a tank is a waste of time. It will not make it give you more pressure.
Compressors are not all that dear to buy, so I advise you to buy a new system to suit your needs. Not modify something.
Think of what you might need and then add 25% to it.
And always think safety first. Get your self a good respirator as well.
Regards and respect