Add work email address to Gmail on an Android device
The Exchange option is only available for users with the Exchange Services package installed.  Unfortunately, this package doesn't come installed on all Android phones (as far as I know, only the Nexus devices come with it preinstalled) and isn't available on the Play Store.  However, I did find a link where the apk file can be downloaded from.  After installation, the Exchange option should appear when attempting to add an account (reboot possibly required).



Disclaimer: I myself have a Nexus device and already had the package installed.  I have not attempted to use the link included in this answer and cannot vouch for its safety.