The Add Site System Roles Wizard can be run from the Administration\Site Configuration\Servers and Site System Roles tab. The first step is to mark the chosen server and chose the Add Site System Roles option.
In this particular case, there is no need to point to a specific server name on which a role will be installed or assigned to a site; as this data is automatically populated when choosing an existing ConfigMgr server already assigned to a site, and this has been done in this example:
Installing a server role on a server already existing in a hierarchy
The Site System Installation Account option is used to provide the Active Directory account that has appropriate permissions on the remote computer needed to perform an installation. Further steps look exactly the same as when choosing the Add Site System Roles option.