By all means, go ahead! We fully support modding efforts from our community, and are always happy to see people customize our products to their needs and likes.
All objects generally are offered with modify permissions, whilst scripts are copy/no mod/no transfer. This also includes our new Avatar Core mesh body lineup, so even though it may list as no mod, you can in fact modify the prims as needed.
For any textures, please refer to the according product manual, all our recent products come with a download link for the source PSD files, allowing you to use Photoshop or Gimp in order to modify or create your own textures.