Adding an SMPP server is the same as for the email-to-mobile SMPP server (see C.5.5.3 An SMPP Server).
The remote SMSC needs to be configured to route SMS traffic to the gateway SMPP server. To do this, the SMS destination address used by the SMSC to route mobile-to-email traffic should be the value set for the gateway profile option SELECT_RE.
For example, if the SMS address 000 is to be used for mobile-to-email traffic, then the SMSC needs to be configured to route traffic for the SMS destination address 000 to the gateway SMPP server. The gateway profile should use the option setting SELECT_RE=000.