Heres how you can.
Firstly, I recommend that you backup your data!
On the KVM host, create the 'drive' with either of the following two commands
qemu-img create -f qcow2 /path/moredrivespace.img 10G
dd if=/dev/zero of=/path/moredrivespace2.img bs=1 seek=10G count=0
This is where it gets interesting.
Now to add your new 'drive', to your KVM guest.
Run the command 'virsh'
Welcome to virsh, the virtualization interactive terminal.
Type: 'help' for help with commands
'quit' to quit
virsh # attach-disk KvmGuestDomainName /path/moredrivespace.img vdo
By default or by specifying the target of vdo, the default driver will be
Virtio. If you are a FreeBSD user like I am. You will struggle, for FreeBSD will not see the device. I find you need to use a SCSI device. And inorder to use / do so, you need to run this command.
attach-disk KvmGuestDomainName /space/moredrivespace.img sda
I also found for FreeBSD, its best to use the dd command.
As per usual, Linux guests just work.
And then reboot your VM.