Mail doesn't import contacts. It searches for them in Contacts as you type.

The only contacts archive Mail maintains is Previous Recipients.

You can export an address to Contacts with Message > Add Sender to Contacts.

You should have been able to import the contents of your original Contacts app when you drug & dropped.

The new Great Equalizer is the SEND button.

In Memory of Harv: Those who can make you believe absurdities can make you commit atrocities. ~Voltaire