If I'm thinking of the same tileset extractor program, yes, it injects the palette into the image, so you should be able to see the palette while viewing the image with any half-decent image editor. The images will be in .png format, though, which JCS doesn't understand, so you'll need to convert them to .pcx or .bmp or something before recompiling.

What sorts of tiles are you looking to add, anyway? If you're looking at the official tilesets, they all(?) have various conversions and expansions that add all sorts of miscellaneous stuff.