Well the problem with going from Square Pixels to 0.9 Pixels is that you’ll lose resolution either way you go. That said your options are as follows:
1) Change the Image Size of the document to 111% width and 100% height. At a 0.9 Pixel Aspect Ratio your document will appear to be 111 * 0.9 = 99.9% of its original width.
2) Create a new Photoshop document using the NTSC DV preset (720×480 0.9 PAR). Make sure Pixel Aspect Ratio Preview is on. Shift drag the layers from your Square Pixel document into the NTSC DV document, they should automatically adapt to the new PAR.