A woman of any build can be feminine. If you want more curves, work on your muscle tone and make sure you eat well - both amount and health wise. Look after your health and you will be the build that you are supposed to be.
The width of your hips and the size of your behind are limitted by the width of your pelvis. Putting more weight on could enlarge them to some degree. But you can make them more shapely at their natural size by toning your muscles - i.e. if you can't make the curves much bigger you can be curvier in shape
Perhaps better health will give you a fuller bust. Perhpas overall you are skinny because you are not eating enough or eating right or your muscles are not developed enough (from exercise).
Femininity is about other things than shape. Your movement, posture, dress, speech, personality, behaviour etc etc. I have seen some very unfeminine women with curves. I have seen some very naturally thin girls who are very feminine. There are attractive women of all builds. It's what you do with it that counts.