I kinda did this in my current campaign. 3 years ago, my gaming group consisted of me and two friends, and we really needed a third member to the party. One of the players had a best friend in his background, which became my "PC". However, I made it clear that she was really an NPC, not my character, per say.
But once 3E came out, so did the players from out of nowhere. now the group has 6 players (with a couple more waiting in the wings if anyone leaves), she kinda had to go. Problem was she was pretty intigral to the plot, so she's become an actual NPC (soon to become one of the party's main antagonists, which I think they're beginning to realise...)