A ten digit number that is hard to guess sounds a lot like a phone number and area code.
A ten digit numerical password probably won't be cracked on steam, but not because of it's length. A low end computer could probably guess every combination in a few minutes. It's the fact that after the first 100 tries valve is probably going to block the connection trying to guess thousands of passwords every second.
If you do some thing simple like throw in "a*" at the end of random numbers the time it takes to brute force will probably go up exponentially since now the computer has to guess letter, numbers, and symbols.
I tend to put a random symbol at the end of a password because of this.
Of course you're hundreds of times more likely to have your account compromised by phishing, forgetting to log off public computers, keylogging, psychological tricks or simply sharing with a friend then having someone guess your password. Familiarizing yourself with phishing sites and an ounce of prevention will go quite a bit further then putting a few letters at the end of your password.
