The tunnel entrances would be sealed and the entire tunnel pressurized.

To expand the base, the tunel would go deeper into the regolith. Especially true if subsurface water ice is found.

I wonder, is the sub-surface ground tempature of the moon similar to that of Earth?

Just about anywhere on earth, the year-round ground tempature is 55-60 degrees F. if you dig deep enough...Dig deeper and the tempature starts to increase, along with pressure, proximity to volcanism, radioactive elements, etc...

I've heard that NASA has had contacts with the US company Caterpillar that has many earth moving equipment lines.