Ideally, I would do this during the OS installation, but you can still use the command lines to create additional volumes, see guides like this.
Note the WebApp only recognise block devices in the form of "/dev/sd?", and not volumes. So you'll need to manually enter the volume paths in the media text box.
I have a ticket for adding LVM detection in the WebApp, this will be added in the next update.
Let me know how it goes.