Since you can use several protocols with Pidgin, you could have the same person in more than one protocol. You can group these contacts to appear just as one selecting one of them and pressing the right mouse button. On the popup, select “Expand”. Now drag the other contacts to the first contact. You can also move the contacts around to select the default protocol for that contact.
On the recent releases, you can simply drag one contact over the other and Pidgin will expand it automatically.
Note that the same process also work with Adium.