X-Tension (2000) was released as an expansion to X:BTF; however, many gamers and reviewers consider it more a "sequel" than an "add-on". It follows on after the story in X: BTF, with "the evil Xenon vanquished" and the player "still stranded light years from home." The player once again takes control of Kyle Brennan, who now has "time to kill" as his X-perimental ship is reverse-engineered. The player is given a ship, some credits, and left to make his own way in the X Universe.
Source: Wikipedia, "X (game series)", available under the CC-BY-SA License.