How can you sperm in your dreams?

Technically the correct terminology would be ejaculation (the release of fluid from the urethra). But yes, a male can ejaculate when he is dreaming. They are called wet dreams.

The correct name is nocturnal emission. Also sperm is not a verb so you don't say 'how can you sperm.......' The correct verb is ejaculate so you say:

'How can you ejaculate in your dreams.'