Is this a muscle that, if trained, can increase the width of the waist circumference, or is that determined just from the bone?
External obliques and yes.
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Yes but why do you want to increase waist size? Typically the more taper the better and you would be looking for definition over size.
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I must ask. Why do you ask?
Do you want more pronounced obliques?
Do you want small obliques?
I will say when I judged bodybuilding contests, in most all cases, a “wasp” waist made the overall look extremely weak.
I had heard the physique described as a frame around a picture. The picture is the abdominal muscles. Every other muscle made a frame around the picture. Obliques are the sides of the frame.
Never confuse fat or “love handles” as obliques.
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I have a very narrow waist, since i’m 165cm, my waist it’s 67cm, but I’d love to reach at least 73cm, which would make me able to wear anything good.
Also get stronger there, Will make it look really better!
Since I saw that Franco Culumbu has 76cm of waist, with my same height, I thought there are some muscles we could train to increase the waist circumference.
Also because i think this will give me also more room to play with chest, torso, and shouder circumference.
And also because I have large hip.
And yes, I also don’t love the narrow waist! I think 4 to 6cm would really be awesome to increase!! 
Which are the correct excercises to make it work?
That is small. You didn’t mention how much you weighed. That could be helpful, but with that waist size I would guess you are fairly lean.
First, is that your genetics determine how much muscle you can add various places and your midsection is not an exception. As for exercises, I would recommend lifting heavy weight that involves your core muscles. Those will make your other muscles grow also. (Deadlifts, squats, cleans, overhead presses)
As for targeting midsection muscles that involve obliques, I would say decline situps with your hands clasped behind your head and touching your right elbow to your left knee, and vis-a-versa. Also do side bends with a single dumbbell. Start fairly light and work up to fairly heavy, where you stop at 10 reps with at least 5 more reps “in the tank.” (obviously do both sides.) You should do these heavy oblique focused exercises twice a week.
Others might have some better suggestions, so consider those.
Do know that not all people can achieve nice obliques. One of my favorites is Ron Teufel
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