I have a set of magnesium wheels for my racecar, but the locking holes on the back of them have a different bolt pattern than what I'm using.
Can anyone tell me how I can fill these holes with magnesium? Or can I use aluminium?
Don't have any experience with casting, so please do a step by step.

Thank you

Yikes. Stop!

You can't fill them by casting, you would have to build them up with weldment or use a laser welder in an inert environment, and even then they would likely not be safe! Don't mess with this. The heat stresses alone will do bad, bad things. GET DIFFERENT WHEELS before you kill yourself or bystanders.
I used to race myself, I know it sucks and can be expensive, but you have to get rid of wheels when they are damaged. Don't repair or modify them, I have seen serious destruction and injury from people who thought they could modify or repair Mag alloy wheels, even from simply just from attaching spacers to change offset.