Is there a way for the user to access UWL without defining a portal role?
No, you need the UWL iview linked to a role
Can we connect a UWL user to the backend SAP system using his backend access only?
The user accessing the UWL gets the work item info based on his backend access only..do you mean something else
Is there a function module or BAPI we can use to get a role defined to that UWL user?
yes, there are RFCs/BAPIs which can get User info, roles and authorization from backend..I think its start with BAPI_User*