There are a number of things that a principal can do to make the new potential irreplaceables feel welcome right from the start. It would be great if the principal would take them to lunch before the school year starts. If someone could go to the restaurant ahead of time to leave gifts such as a school t-shirt and personal business cards at each setting would add to the welcome feeling.
Another idea would be to have an Orientation meeting specifically for the new teachers and answer any questions they may have as well as let them know of opportunities that are available for them to grow professionally. It could be helpful to have a panel of teachers who are relatively new to the school to share what they wish someone had told them when they first started to work at the school.
Assigning a buddy as well as a mentor can also help the new teacher navigate the new environment and feel welcome.