Thank you Sdeters, your way is just what I'm looking for.
Since few months ago we've worked with Solidworks, and it has a function called weldment profiles: I have just to design a 3d wireframe, then choosing the section ok the element to place into the wireframe and the joint cut type, so to obtain the frame I need, something like this:
The frame is just a part, and the function even provide a cut list, so I save in my vault the frame with the cut list of the used elements.
I'll try to follow your way in NX, that seems to be very close to what I'm used to 