I've taken a small brownie oven pan and lined it with foil, and then got a piece of lump charcoal nice and ashed over. Put the burning lump in the pan, and then you can put chips on top of it.
It works fine, but I never really noticed any augmented smoke from it, even when using a neutral wood pellet. Perhaps I didn't go big enough, but I didn't want to introduce a lot of extra heat.
I haven't tried mixing chips in a pellet tube, that seems like a much better idea actually.