Multisite – make a blog after user registration

I have a wordpress blog with some users, but I turned it into a multisite a little later. So I would like to know if there is a way my users can create a blog from their account after they have registered as users ?

Right now, to get a blog you need to choose either a blog and username or just username only.

So I would like to automate the process for my existing users to create a blog.

Hope this makes sense. Thanks for reading ??