Most of year, we allow writers to start as many personal challenges as they want on as many novels as they want. But since November is National Novel Writing Month, the biggest writing event of the year, we only let people work on official NaNoWriMo challenges. That means you can't start a new novel or a new personal challenge during November, including challenges that overlap with November in any way.
However, the rest of the year, you can start a new challenge from your Dashboard, or from the Challenge box of any existing novel.
In the meantime, if you want to work on another project during November and you don't already have any old ones created, you could start taking notes on it by adding a "Note" to your NaNoWriMo project.
Also, if you meet your goal super early in the month, it probably means it was way too easy. You should raise it! Check out the other "How It Works" post labeled "How do I edit my word-count goal?" in order to do that.
When the NaNo challenge ends on December 1, then you'll be able to create your own personal challenge from your Dashboard. Your NaNo novel will also be stored on site and you can keep working on it whenever you want.