Currently, it is not possible to define individual workflows for each site in multisite. We have a feature request logged in our feedback portal and hopefully we will be able to implement this functionality for our future releases. Here is a link
where you can track the progress of the feature request and vote to increase its popularity.
I apologize for the inconvenience caused by the lack of this functionality.
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Sitefinity CMS Ideas&Feedback Portal
and vote to affect the priority of the items