Can circumcision be undone? can doctors reattach foreskin?

No: No. The movement to "reclaim the foreskin'" strikes most of us in the practice of urology as somewhat bizzare. There are skin stretching gizmos available but I am uncertain that one could reasonably expect to stretch the remaing penile shaft skin to resemble an uncircumcised penis.

Not really: Foreskin is gone ; thus can not be replaced. However, absent foreskin can be replaced with a skin graft. This is quite major surgery.