If an achievement isn't in your Available Achievement Section, then you can't get it because those require prerequisites.
The general exceptions to the rule are Event Achievements.
And, as of right now, the Timid and Ultimate Player achievements which are truly hidden.
Actually, I just got the Timid acheivment, and I think I remember seeing it on the avaialble achievments section. How I got it was going to quote, and deciding not to a few amount of times. VERY easy acheivment
You get the Ultimate Player achievement by having a lot of friends of the opposite gender
You mean, you require a certain amount of friends on your list? Well that sucks. I like keeping my list as small as possible...
You have to have at least 50 friends, and the majority (but not all of them) must be of the opposite sex.
If you're a cheap achievement whore like me, you can attain this by hanging around the Achievements forum and asking people to temporarily add you for the achievement, and then remove them when you're done.