The Victorian process is a foreign to me, but the general principles probably apply. An addendum to a will, often called a codicil, usually requires the same execution standards as the original will - i.e. Signed by the testator in the presence of two witnesses. So, unless your mother made such a codicil before passing away, you’re not going to get one.
What you’re actually going to need to do is make an application to the Supreme Court for rectification of the will and obtain probate (you’ll need to complete the probate process before the Court will look at making an order for rectification). Technically you could do it yourself, but it’s likely to be a bit of a daunting process. It may also require some form of evidence from the person who drafted the will in the first place, if available. I’d suggest finding an estates lawyer in your area. You may even have a claim against the person who drafted the will to cover your costs if it can be shown they were negligent in the drafting (as opposed to being given the wrong names, making sure the names were checked by your mother prior to execution etc.).