Here in Quebec, we don't need front plate and since I'm considering importing a car from USA or rest of Canada, I was wondering how the front plate holder is fixed? I don't like the idea of a front bumber with empty holes...

 

you can always get the show and go plates

 

you'll need to patch them up, it's a pain but not too difficult assuming you have a touch up pen

 

I've heard of people getting little caps that go in the holes. They color match the paint and it doesn't look too bad.

Something like these:

 

Can you use the show n go frame with a stock bumper or did you have to get an aftermarket because i thought it hides in the bumper lower grille

 

The bumper plugs are very easy to DIY here is the DIY I used easy as pie...
DIY: $10 Bumper Plugs

I used 5/16" plugs instead of the 3/4" plugs the OP used as my holes must not have been as large as his, I just measured my holes with a drill bit to determine what size plugs to buy

And the larger rectangular piece is called the "tow hook cover", just purchase one and have it painted...

Here are some before and after photos...

 

i kept mine on incase some asshat decides to back into me...

 

$10? my god. jus go lowes $.50 lol