Right now the extension is able to provision an user that does not exists previously in any other store, but not to link an existed user on the specific store.

Take in mind that each websites is consider as unique Service Provider able to be connected with a Identity Provider. It seems that in your scenario, you are connecting all the stores to the same IdP (not the common use)

I can implement the ability of link an existed user to the specific store, but I will need to add an extra option on the settings to enable it because in other scenarios, where you connect with different IdPs, this can provoke a security issue.

P.S Do you have license for all those sites?