It might be possible to seal a hairline crack. But finding someone who knows how to repair a residential steam boiler will be difficult. Your best option is to keep the water level low. What is the water level on the sight glass where the boiler begins to leak water? Does steam escape when the boiler is operating?

Thanks Mike. We were keeping the boiler low, and the plumber that works with the contractor ended up recommending boiler cement for our short term solution. Water is milky now and we understand that the cement can gum up the radiator - so we are back to keeping the level low and it seems to be holding and hoping the cement doesn't complicate things!