If your water pressure in the whole house was 10psi, you'd know it! Water would trickle out of the faucets and your shower would be a dribble.
Every boiler system has a minimum pressure it requires, and most (if not all) have a safety cutout switch to stop it from firing if the pressure is too low. The manual should say what you need...10psi may be a little low. Just like in a car's radiator, you need some pressure to prevent the water from exploding into steam.
I seriously doubt your boiler is directly connected to the house's water supply without something in between. There has to be a valve of some sort, and often, there's an autofill valve (but it sounds like you don't have one of those, or if you do, it is either defective or misadjusted). There should be a backflow preventer between the boiler and the potable water supply, otherwise, you could get stagnant, metal saturated, nasty water back into your drinking water.
You may find, if the boiler will fire, that the pressure increases when it is running because the water expands. I'd try that first before I added any water to the system. The expansion tank tends to limit that rise, but there may be some.
Last thing I can think of, the pressure gauges aren't always that accurate and sometimes stick a little. Try tapping it and see if the pressure changes.
Having a screw-on pressure gauge so you can check the house pressure isn't a bad idea to have around...typically in the $10 range. Get one with a second, tattle-tale hand that will show peak pressure over time. You can use it on the boiler, too, attach to one of the drain ports, and open it. That would help confirm whether the gauge on the boiler is working right.