Mail: Can I manually add (alias) labels?

I've decided to switch from Gmail to iCloud. I'm currently using several Gmail accounts, each representing a specific category of mail (e.g. YouTube). I've discovered that iCloud offers aliases, and I thought it would be a good idea to use a different alias for each category. That means I can just use one account, but still have a nice seperation of categories. Luckily, 3 aliases is just enough, although I wish iCloud would offer a little bit more.

In any case, I was positively surprised when I discovered that iCloud Mail also gives each message of a specific alias a nice label. And that leads me to my question: the labels are cool, but are only given to messages with the right alias. It would be very cool indeed, to be able to import my Gmail messages to iCloud and manually add the label (e.g. YouTube) to the messages. Would such a thing be possible to do? Thanks in advance.