Lighting, Light Fixtures, Ceiling and Exhaust Fans - J- Box or No J-Box

I am installing fluorescent lighting in my garage and one of the current cheap single bulb fixtures is too close to the attic access to mount my 4 tube fixture centered over the box. Is it wrong to add a 6" length of wire and "snake" it over to the center of my fixture even if it is contained under the fixture? If I need to add a new box can I just replace the old box with a J-box in the attic since I would want to repair the hole? are all J-boxes the same? it would be covered with blown in insulation and I don't want to cause a fire hazard.

Thanks for your help.

Yes you may add a length of wire. The wire must come out of the box in the same manner as the existing wires.
You do not need a box at the new location, the wire can go directly into the fixture. Use a RX connector at the fixture. These fixtures are considerd the "box". make the splices in the existing box and buy a blank cover for it. You may not bury the box--ever.

You can certainly install a junction box in the attic, as long as attic is accessible. Move the box above the insulation.

But this is a garage for gosh sake, just cover the old box with a blank cover, painted the color of the ceiling if you must.

Thanks for the help. I'm not familiar with what an RX connector is though.

He means Romex connector, but really he means NM connector since Romex is a brand name.