Sidestepping the comments about whether or not Namecoin is a fork or a separate project or whether bitcoin is an investment or a currency, I'll address the direct question asked: Is there a way to easily and automatically purchase several crypto-currencies at once? Not at present time, no.
Bitcoin (and its forks) is a very young project, though, and I'm sure there was a stock exchange before DJIA or S&P existed. Give us time and it's very likely that something of the sort will be created - especially now that the demand has been made publicly known.
Namecoin is not really a BitCoin fork. It is intended to work like DNS, not like a currency. As to the other "forks" I would highly recommend you not take seriously digital currencies which lack the community backing of bitcoins, and are not heavily feature-differentiated from them, until or unless they prove themselves popular and reliable. Even bitcoins themselves hardly meet this criteria, so that should give you an idea of how seriously "IXCoin" and such should be taken.
But the broader problem here is that you seem to be approaching bitcoins as an investment. Bitcoins are not an investment vehicle--they are a currency. There is no need to "diversify your holdings" because bitcoin is not a holding--it's just money. If you don't need it to store your wealth or exchange it for something else, you're looking in the wrong place.