A client chat application that is built using Smack may connect to a XMPP server that offers the ICQ transport. So this client can subscribe to the ICQ transport service and make use of all features available in that service (e.g. sending SMS via ICQ).
E.g. using a JAJC (Just Another Jabber Client) client may connect to the “amessage.de” XMPP server to send SMS via ICQ transport.
Therefore, your Smack client will have to provide the GUI for end users to subscribe to the other transport services offerred by the XMPP server (namely, passing the login information for ICQ, Yahoo!, MSN, AOL, etc. to that server).