There is a tedious way to do it, currently.

1- Log in your alt, go to some one-man instance (class quest, whatever), then log off inside said instance.
2- Log in your main char, invite your alt to your friend list.
3- Log in your alt and accept said invite.
4- Profit.


I know there should be a normal way to do it, without tricks like this. But, I just wanted to note that doing what you request is already possible, in case you didn't know it.