hmm i use two ways to do so. the old one is to put the model to front view so i can see the bow directly, then i add a cylinder or whatever form of the tube door you want, position it in x and z axis and move every vertice towards the bow, until it is positioned upon the surface. then i select the two neighbouring verices of the bow shape and an edge vertice of the tubedoor and create a new triangle. when it is done, i smooth the surface by unwelding the tube and the bow, this creates two objects and the penetrated bow schabe will need furter correction of the edges around the hole. or i simply add the boolean plugin in milkshape, create my 3-dimensinal tube and make it cut into the bow in the desired position, then i make a boolean operation that automatically cuts the hole which i just need to clean from surplus vertices afterwards.do not know how boolean is done in wings3d, but method one should work...
see
https://imageshack.com/a/utnz/1