1.- You can't. Em is a variable measure used in CSS based on the width of the letter m of the font that you are using, therefore its value will depend on which font you use, and the size of that font. Photoshop only uses fixed measure units.
2.- Again, you can't, for the same reason explained above.
On the other had, you don't need to. Create you picture in photoshop using px, give it the size that you think it will be used in most monitors...and then, when you create your web layout, do not use pixels in the img element, but em. It works very well.
Before asking, do a search... if you don't find the answer, then ask The purpose of this forum is to help others in the community, that's why it's called Sitepoint and not Linkpoint. SP Guidelines - No fluff.