@okh: thanks for your explanation!
So I got it right, that in AOI ONE physical part can only have ONE texture and I can not apply different textures/colors to the sides of the same cuboid?
Than I'll have to also model the doors - not a big deal, but I'm really a newbie and being used to "real" and ...

@Puybaret: thank you very much for your quick reply!
I do have to apologize again - I think I formulated my question not very well:

Let's assume I have imported a simple cuboid with a given texture (wood), created by AOI and saved in OBJ format.
This would be a very simple dummy for a wardrobe.
To ...

I'm sorry, if this should be a trivial question, but I wonder:

How can I draw or paint something directly on the surface of an object?
Is it possible to create a rectangular bitmap or vector graphic and "glue" that on the surface of a cube or another object in AOI?

thanks a lot and kind regards ...