Are you going to run your kitchen hood exhaust through your HRV? I don't think that is an approved configuration because of the grease in the exhaust. Let's assume you don't for the 2nd part of this answer.
An HRV tries to recover heat from air exhausted through the HRV and use it to warm incoming air. When running your kitchen hood, no air will be exhausted through the HRV so the HRV will be acting like an open window: fresh air will come in through it but it will not be warmed.
During a cold winter, I assume you would find this situation (cold outside air distributed through your house) to be uncomfortable and undesirable.
All the literature I have seen assumes you want to take the cold outside air, hundreds of CFM, and heat it up to 70F so that you can turn around and blow it outside. Equipment capable of doing this is expensive and it seems wasteful of energy.
In my house, I have two windows right next to the hood and I just open the windows when I use the hood. Outside air comes in, mixes with the cooking fumes and goes right out. Standing in front of the range is a little colder than the rest of the house but at the same time, you have the heat from the range so it's not a big deal in my experience.
The bottom line is to consider if you want to heat your makeup air and if not, bring it in as close to the hood as possible, but in a way that helps capture the cooking fumes and doesn't just bypass the range.
You cannot cover any junction box that still has live wires in it. Your best bet is to either remove the box all together or just put a cover plate on it. Your other option is to run a new circuit to the new box and de-energize the circuit to the old box and mark the source wires as "NOT IN USE" at your panel in which case I believe you could cover it over.
If there are live wires in the box, sooner or later you will need to get to the box again, which you can't do if you've covered it over! You'll thank yourself later (as will any future home owner).
Best Answer
It's common enough for range hoods to be supplied with a cord and plug. It's also common for range hood connections to be made via BX.
The range hood should be designed so the cord-and-plug connection can be removed, presumably exposing a standard 1/2" trade size knockout (more like 3/4"), and then the BX can be fit up to that with a BX fitting. There'll be a compartment that opens up to hook up the wires.