Can we convert our gravity water system to a pressurized one? Is there lot of trouble and expense to so?

Hi
“Good Part” Normally it is not a “big deal” to have your system upgraded and to add a pressure pumps. On average it will take a plumber 1 day to do it and somewhere between 500 to 1,000 US $ in cost (depending on the type and size of the new pump.
“Bad Part” this operation is relatively cheap and easy if your plumbing is in good shape and can take increase of pressure. All the pressure pumps sold here are set to about 40PSI which is about a half what you will get coming out of the faucet in US, Canada or Europe but enough to burst the old pipe or cause leaks.
For that reason have your plumber to check if such upgrade is logical in your particular situation or you may end up with pressurized system as well as re-plumbed home.
Adam