Keira doesn't reply in writing.
She does handsign any pictures you may send along if she has time, though. There's generally no note or anything, they will just be sent back to you.
Whether she reads fanmail is anyone's guess. It might be that someone at her agency picks out a few for her to read every now and then, but there's no way of knowing. I'm pretty sure she'd run out of time to act if she were to read all the fanmail