How to make a host user to access all meetings.

Yes, you can add them as an individual or as part of a group to all the meeting rooms and then give them or the the group Host permissions. You should also be able to do this programatically via the APIs if you didn't want to do it as a manual process.

Keep in mind that just because they are listed as a Host of the meeting room, doesn't mean they will be able to start and run the meetings if you are on the Named Organizer model. A member of the Meeting Host Group must be in the meeting room for it to activate and run.