role assignment

Is it possible to change the role assignment after the process have been already started? I tried to "use an expression" for assigning the role to the task of the process as a potential owner.

1) I tried to write the expression as follows,

getPrincipalByUniqueName(PotentialOwnersInternal/principal/principalId, "approver")

"approver" is the role that I defined in the UME and is included the several users.

But, this expression is wrong, because the prindipalId is the list of xsd:string, this function expects the type of xsd:string.

How should I recorrect the expression?

2) If I can use an expression like above, If I add a new user to "approver" role after the process have been started, can the new user execute the task that are assigned "use an expression" role as a potential owner?

Best regards,