I solved the problem. It's a very simple solution.
With ent_next I'm able to iterate through the list of entities.
I'm using XML-Serialization to save all textures (a list of strings) into a file - 'mapname.tex'.
After saving/closing the map, every texture will be saved into 'mapname.tex'.
By reloading the 'mapname.wmb', the application will search the '*.tex' file of the map in order to reload every single texture with an ent_next loop.