Lots of things doen't like to work with that low pressure! I'd think I'd rather raise the house pressure, then things would work better, rather than just trying to do this for the boiler.
A boiler makeup water needs a backflow preventer to protect the potable water from the stagnent stuff in the boiler circuit, so you can't just leave that path open.
This is a creative guess...if you isolated the expansion tank, lowered the pressure to say around your supply pressure, then filled the system with what you have. If you then closed off the supply and pressurized the expansion tank by pumping it up slightly to the needed system pressure, you'd probably be okay. If there are leaks, you'd have to do this periodically, but since the boiler circuit is closed, once you fill it, it should stay.