Go to the SELECT MENU -> choose SELECT ALL [this will select both the new inner frame and the image for copying].
Go to the EDIT MENU -> choose COPY [this will copy the bordered image]
Go to the FILE MENU -> choose NEW. To create a 100 pixel mat on each side and top and 200 pixels on the bottom, I change the 504 to 704 [as before] but now I change the the 364 to 614 [instead of 564 as above in Method A]. This makes a new photoshop window appear. As before, I now paste the copied image into this window. go to EDIT MENU -> choose PASTE. However, the image will be pasted in dead center. So, now I have to move the pasted image up. To do this I click on the move tool and either drag the image to where I want it or use my arrow keys to move the image. The arrow keys are more accurate. Here's what the move tool looks like.