Yes it is. Just contact Optus directly and they'll set it up. (Facebook, Livechat, etc.)
Note do be careful using RSP supplied email addresses. You will lose them if you ever leave Optus.
A cloud RSP like GMail will not be affected by which RSP you are with.
WinnersOfOz, you've posted this in the Optus Sport forum... just in case you mean can you change your registered username for Optus Sport, please shoot the team a message for assistance with this.
Otherwise yes, follow Peter's advice 🙂