Gluster users - Aug 2016 - Convert replica 2 to replicat3 (arbiter) volume

Hi,

I just finished to read the documentation about arbiter
(https://gluster.readthedocs.io/en/latest/Administrator%20Guide/arbiter-volumes-and-quorum/)
and would like to convert my existing replica 2 volumes to replica 3 volumes.
How do I proceed? Unfortunately, I did not find any documentation to that
precise topic.

Also I was wondering if it is recommended to use a virtual machine for the
arbiter instead of a physical machine. I suppose that in a replica 3 if the
arbiter is slower than the two other nodes then the whole volume will suffer, is
that correct?

Regards,
ML