You can :) The process of getting a normal/specular onto your model is very simple. Sunburn has an editor that can be used to drag&drop texture files into the material that is used by your model.
I'll try to sketch it using words :)
If you import your model, it will have a material attached to it (if this was exported with the model) this material can be used inside the editor to add diffuse, normal, specular maps etc. to the material updating your model right infront of you with the new maps. It's very cool to tweak your materials and instantly see the result in the actual game.
I hope this clears things out for you.
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