I, located in Canada, bought UL-listed surface mount ceiling spotlights with diameter of about 6.5cm from Italy. My builder says it is illegal to install the cylinder body directly to the ceiling surface like indicated in the instruction. My ceiling is prepared with regular junction box. Could anybody help me with this please.
The one thing you cannot do in the US (Canada's electrical code is very similar to the US's and I'd imagine is 100% in agreement on this), is to install this without an electrical box by putting some sort of wall anchor in the ceiling as shown in the instructions. Your electrician was 100% correct on that statement.
To install this fixture, you'll need 2 things:
A way to hold the light onto the ceiling. Often, a cross bar that you will attach to the screw holes on the back of the fixture then mount the bar to the two screws on opposite corners of the box is supplied with lights designed to go on US boxes.
A rose or cover plate that will cover the parts of the junction box that are not covered by the light fixture itself. If the manufacturer doesn't supply them you'll have to come up with something that will work.
Something like one of these might work:
One of these would be great. It's plastic and would be easy to drill the right size hole through. However, I don't believe you're allowed to modify it and still have it be within code
This one might be your best bet. You can knock out the knockout to run your wiring through, then drill the appropriate mounting holes to actually mount the fixture to this, then use the mounting screws on the box to mount the cover & fixture to the box. It would, in effect, cover requirements 1 & 2 at the same time. You can modify metal boxes for mounting like this (see the 5th paragraph of this answer).
Of course, any of these could be painted to match the ceiling or fixture.