What would be a good 67 mm lens attachment for my camera for football games?

I have the nikon d50

Nothing.

You need a relatively fast 200 or 300 mm lens to shoot football.

Adding anything to the front of your lens will only decrease that amount of light your lens can transmit to your cameras sensor

Here is a sample shot, using a 300 mm f/2.8 lens

This photo was shot at 3200 ISO at f/2.8

Your cameras highest ISO is 1600 so already a stop slower. Put a 18-200 mm lens that is f/5.6 at 200 mm, subtracts another two stops. Trying to shoot anything under those condition's will create blur like a wildman just from the camera movement, not including the movement of the athletes

You may want to reconsider your equipment.

Even if you had a $6, 700 300 mm f/2.8 lens on your fine D50, you would be limited by the top ISO of only 1600

If on the other hand, you will be shooting during daylight hours, the new AF-S 70-300 mm lens will do the job, but forget add-on filters (called lenses by the sellers)

The add ons will degrade your images so much it's not worth the money you pay for them. On top of that they will often render your auto focus and cameras metering inoperable.
Adding to ace's night time… You really are SOL with your camera. It just can't do a whole lot, if anything even under the best lights. Our school has awesome lights and this was shot at ISO 12800, f/2.8 and a shutter of 1/500

Notice the horrible noise in it? It will be 10 times as bad as that if you try to raise your exposure in post processing. Totally un-useable.

None. Just shoot then crop in the computer later.