Steaming generally doesn't make meat juicier -- it is just as easy to dry out a piece of meat with wet heat as it is with dry heat, if not easier.
There are two things that you can do to make your turkey juicier. The first is a brine, which Elendil suggests above. The second is to make sure you aren't overcooking. I'd suggest using a probe thermometer and pulling the tenderloin when it is at around 160F and letting it coast to your target temperature of 165F.