I get asked this a lot.

Yes, you can add a 40 barrel to your 10mm pistol and it is pretty easy.
Here are some pics of what I did to add one to my Colt Delta Elite.

First, find a barrel. This is pretty easy depending on your pistol. Here are a couple of sources:
1911 5" - Numrich (Kahr)
1911 4.25" - Brownells (Storm Lake)
Glock - Lone Wolf Dist.
Tanfoglio - EAA

I bought one from Numrich for about $80. It is a very nice 416 stainless barrel. The crown is actually nicer than the stock barrel:

A 1911 is old school (i.e. not modular) so it requiries fitting. It should be minimal and any gunsmith will be able to do it. Other platforms probably will not.

I was going to buy the Wilson video, watch it and fit it myself. However, I sat on the barrel for about 3 months before I decided to just take it to my local smith and have it fit. It took him a few days and cost me less than $50.

Here is what it looks like:

What about mags and springs?
Good questions.

I put it in a 20# Wolff with a standard guide plug that I already had.
I also used the stock 10mm magazines and it has worked perfectly with several brands of ammunition.

If I was to rely on it for self-defense --not sure why I would underload a 10mm--but I would get a couple of 40S&W 1911 magazines with the spacers. The OAL differences of the two cartridges is pretty great so I would be worried about potential feeding problems. Since this is just to use cheap(er) range ammo, it is not an issue for me.

Pretty neat setup for minimal cost.