How to add, additional prismatic joint to the end of Universal robot last joint inside URDF file?

Could you please tell me how can I add, additional prismatic joint to the end 6th joint in URDF file of an Universal robot as in the link below:

https://github.com/ros-industrial/universal_robot/blob/kinetic-devel/ur_description/urdf/ur5.urdf.xacro

The last two links (wrist_3_link and ee_link) are connected by a fixed joint. Change the joint type to prismatic and then update the joint properties (joint axis and joint limits).
Or do you want to add an additional link to the manipulator i.e. add a new link after ee_link? In that case, define a new link with a unique name, for eg. new_link. Then, define a prismatic joint with parent as ee_link and child as new_link.