Supplemental pump

Recently our well pump...down around 400 feet began to slow down and now only gives us 20 psi and runs all the time. I know it needs replacement but could I temporarily add another pump at the end to assist the water up to the top? I know of people with low pressure from city water and have had to do the same thing. I would probably connect it to the pressure switch ensuring they both kick on and off at the same time.

You have either a failed pump or a failed pipe from the pump to the tank. Someone with a clamp-on ammeter could determine which it is: Low amps means failed pump; high amps means failed pipe.

Adding a pump will not be a satisfactory fix because either the pump or the pipe will get worse. In the meantime it would cost you a lot of money for a new pump and a lot of money for electricity to run the bad pump in the well.