RESOLUTION
You need to have the following options to set up Expand environment with Centralized Mail
* Plesk Expand Per Server License
* Plesk Centralized Mail Server Management License
* Plesk Expand Centralized Mail Addon
* Plesk 7.5 Reloaded standard license
You have to install Expand and "Plesk Expand Per Server License" with "Plesk Expand Centralized Mail Addon" on it.
Install standard Plesk 7.5 Reloaded on another server with "Plesk Centralized Mail Server Management License" on it.
Install standard Plesk 7.5 Reloaded with standard license on it where customers' domains will be hosted.
"Plesk 7.5 Reloaded" should be added into the Expand control panel this way:
Expand->Servers->New server
"Plesk Centralized Mail Server" should be added into the Centralized Mail section.
Expand->Centralized Mail->New Mail Server "Plesk 7.5 Reloaded" should be assigned to Centralized Mail
Expand->Centralized Mail->Mail Server->Assign 
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