Just want to know, is there any work around to allocate multiple Roles to a user. The user with multiple roles should able to handle all assigned role's responsibilities.
There is no functionality to allow assignments of multiple Roles in the way that a Permission Set allows extension of the Profile's system permissions.
To allow access to records in other parts of a Role Hierarchy you can use Sharing Rules or Apex sharing based on either the user's current Role or another part of their profile.
Another option is to use Public Groups for the users to allow access to records without being part of the same Role hierarchy.