As an "Admin" you can setup a "Signature" for each user to be used in any emails that will be sent to your customers.
To setup signature for each user, go to the Users in the main setup menu (this is the drop down on your customer name on the top left hand side), and then select Account Settings >> Users. Then choose a User you want to setup the signature. Then click on "Edit Profile" and at the bottom of the form you can see there is a text area box where in you can fill in the signature text. In most cases, you can copy and paste your existing signature from your other Email program. You can use the formatting buttons to edit the look of the signature as well. Click <Update user> button when done.