Is there a way to add a user to "Your Fluther" by username?

Use the search, and search for the username. Click the profile and click add.

Yes. You go to the person’s profile and then click on the “Add to your Fluther” tab.
If you can’t find the person, but know their profile name, type the name into the Search bar at the top right hand corner of the page and find the person there.

I realised i could go to http://www.fluther.com/users/username and add from there. Thanks Guys

I just found a quicker way: http://www.fluther.com/followuser/username/

If you click on the hot link under the person’s name when they answer a question, it takes you to their page, where the Add to My Fluther link is. Also, if you’re on someone’s page and scroll down, you can see their Fluther. The names in the Fluther are also hot linked and will take you right to the Add to My Fluther link. If you’re looking to add former ABers, that would be the most effective way to find and add people.