You are restricted to the built in placeholders, however, you can move them
around in your Slide Master if you want. But you can't add new ones.
SORRY!
No, you can't add placeholders on the Master, but if you fully explore the slide
layouts you might find one that suits your needs. If not, you can set up a
"dummy" slide with the layout you want (using text boxes) and then duplicate and
edit that slide each time you need that layout.