They'll be doing so with Agent Nico DaSilva, as "Hangar 6 R&D" takes place before main protagonist William Carter is recruited to the Bureau of Operations and Command—which becomes XCOM. According to this post on 2K Games' official site, DaSilva appears to be participating in a series of tests.
DaSilva will have a new series of powers and perks, but may also upgrade his path using some of the standbys available to Carter in the main game. New weapons, including an AK-47 and a microwave gun "that cooks enemies from the inside out" will be available as well. 2K Marin says "there's enough experience to level up a couple of agents of every class," so this won't be some one-off series of events.
"Hangar 6 R&D" will be available on Tuesday for $4.99 on Xbox Live. There is more information and two more images at the link.