How to create a custom Precondition to have multiple approvals in the RTC Workitem ?

You can write operation advisors if you need custom preconditions but that seems limited to WI save and delete operations only.

Here is a good link: https://jazz.net/library/article/495 to start with writing custom Operation advisors.

I am sure there are more on this in here.

Here is a workshop also:
https://jazz.net/library/content/articles/rtc/4.0/extensions-workshop/downloads/RTC40ExtPoT.pdf

Else I think you can do a lot of things from Team Configuration -> Operations Behaviour -> Preconditions..as there are a lot of preconditions already provided.

Hope this helps!