The Easy Way: Go into Email settings of employee and do normal forwarding.

or

The Hard Way from Admin Console

1. Admin Console
2. Apps
3. G Suite
4. Gmail
5. Default Routing > Add Setting
6. Recepient = from email address
7. Scroll down to Recipients > Add > To email
8. Choose: Perform this action on nonregognized and recognized addresses

or

1. Admin Console
2. Users
3. Click User’s name
4. Click Account
5. Scroll to Alliases to Add or Remove