Hi, I am wondering is it possible to do this with Your extension:

When user register he would have to get his own subdomain for chosen username. Like this USERNAME.SITENAME.com for example.

Is it possible to do ?

Yes this is possible.
In fact, it is possible to create a specific JMS Layout that do both the creation of the website and also create the Joomla user.

We already did such kind of things.
You can see such kind of prototype with the VM Layout that we illustrated in the tutorial video 14b.
www.jms2win.com/en/tutorial#jms13x-14b

To have an automatic wildcard subdomain recognition, it is required to use a VPS or dedicated server with root login.
We already did that with WHM/cPanel.