DVD handles mix aspect video just fine (in different title sets of course). So just drop them in, the one thing you have to do is decide how the 16:9 will get displayed on a 4:3 TV, the most common is to choose 16:9 Letterboxed, which is what gives you the black top and bottom bars but the video still has the 16:9 look. If you choose Pan and Scan it will center crop the video on 4:3 which is not normally wanted unless graphics and framing has taken this into account.
If you are working with an older version of Photoshop you are off with the canvas at 1024x576 and design your menu (everything will look correct) when done resize the image to 720x576 (it will now look distorted) bring this file into DVDSP and choose 16:9 and it will look correct again. The latest versions Photoshop do this conversion for you, you just start a new file as PAL/DV Widescreen.