Updating One Table from another Table

I have a database that is made by a third party.
I have built a second database basically as a bunch of addons for the for third party.
They have a customer table in their db. (with less fields then i need)
I have a customer table in my db. (with more fields then theirs)
I intend to have a query that runs on open of my database, what it will do is:
Match the two tables up against each other. Update any field that has been updated in the third party db to my db.
It will add any customers that are in the third party database, (that aren't in my db)in to my database.

I hope all that makes sense. I am really not sure where to start. Do I use an update query or append? I want the data to be stored in both places. (only because I have no choice)

Let me know if you need more information.

Thanks

Dan