One of the easiest and most effective conversions to achieve for a beginner modeller (or indeed, and advanced one) is to do something with the wrists of your characters. By changing the angle that a character is holding a blade (or other weapon) at, the entire pose of the miniature can be dramatically shifted and changed.
The change of pose is what I was gunning for here with this conversion. In the image, there is a thunder hammer and a power sword (Grey Knights in this case) which I wanted to connect to a space marine arm and alter the angle of. For this particular character, I had in mind that I really wanted the sword to look like the pose that a fencer would have. Extended, lethal, and in complete control of the weapon.
All this can be readily achieved by a simple wrist rotation based conversion. Snipping off the original bolt pistol from the left hand at the wrist, all I did here was to attach the Grey Knights sword at an angle that wasn't originally there in the bolt pistol based arm. By pinning the wrist, I doubly ensure a good wrist position when the glue dries and a solid miniature that is unlikely to snap (this is important as the blade will no doubt extend significantly away from the body of the miniature).
The image also shows that I've drilled the shoulders and am ready with the good old paper clip pins to attach it to a torso. I think the end result is going to look good here: a tactical dueller on the battlefield ready to take on all comers.