In order to train an officer with a skill they don't have, you have two options:
1. visit Sol Base (or some other bases, but I know Sol has it), and visit the vendor in the Personnel section. Near the vendor to purchase new BO officers, there is another one that will sell skills for all ranks (costs Merits). Click the skill you want, and it'll open a new window that shows your BOs. Click the BO that gets this particular skill, and after a pause, the game will inform you that the BO has learned the skill. Note that game lag can mean it'll take a bit sometimes for this to register. Also note, that until you see that skill, at level 1, in your BO's skill window, DO NOT place points into it. Like I said, there is lag in learning a new skill, and you might lose points if you try to level it up too quickly. <_<;;
2. Train with an Unassinged Officer. As you play, you'll earn new officers from mission rewards, leveling up, or buying them from the Exchange. Until you Commission them into a position, they'll remain on the left side of the Assingments window (chillin' in the Ten Forward, no doubt ). If you like a skill one of them has, but want to keep your current Officer, then Commission the Unassigned Officer, then select Train, then click on the skill you want to train, then click on the BO you already have in a position, and the skill be be transferred over (same rules on lag as above). Note that once you do this, the Unassigned Officer will be removed from your Assingments window (they head back to Starfleet, emo that nobody wants them >_<).