Let's get some terminology straight here so we all know what you are talking about.
The part of the inner tube that sticks through the rim, through which you pump air to inflate the tire is called the "Valve Stem".
There are several different types of valve stems but the type that is most commonly used on cars and motorcycles is referred to as the Schrader Valve. This has a fairly large inner diameter. The inside is threaded and a "valve core" is threaded in that allows air to be pumped in but blocked from coming back out.
The Schrader valve stem has several different varieties, variations of the outer diameter. A "TR-13" is commonly used on automobiles and bicycles, it is simply a rubber coated stem.
The "TR4" or "TR6" is what is commonly used on dirt bikes, it has an 8mm diameter metal, threaded stem.
The threads on the outside of the valve stem simply hold the stem in place in the rim while the tire is being assembled. Once the tire is inflated it really doesn't do much anymore.
The only threads that would have anything to do with an air seal are the inner threads that hold the valve core in place. The valve cores are standard parts, you can go to just about any auto parts store and buy a package of valve cores to replace a lost or damaged core. If the threads on the inside of the valve stem are stripped, however, you will need a new inner tube.
If you buy a new inner tube, be sure to buy one that is the correct size with a TR4 or TR6 stem.