if you generated the atlas with packtextures method, it will return an array of rects to you witch is the uv positions of all textures. if the texture is generated outside unity you should have the starting pixel number and width and height of each texture. then you can get each texture with GetPixels method of texture2d class and put them in a new texture to use.

sry for reviving this but I didn't actually wanna make another thread for this. I know how to create atlas using 3rd patry programs and plugins but what I dunno is how I can create it using packtexture and apply it to unity. they all talk bout scripting and all. one is here by phantom and here by unity manual. unfortunately none of em explained how to use them. I can't even find a single and simple tut on this out there. Is there a simple way for starters to learn how to atlas and use it in unity ? thanks in advance