I understand that you want to have an "Entity Server Block" that has an input port providing the service time duration. I assume you are using MATLAB R2016b. You are able to add an input port to the "Entity Server Block". This can be achieved by following these steps:
1) Double click the "Entity Server Block".
2) In the "Main" pane, choose "Signal port" for the "Service time source".
After the above steps, an input port should appear on the "Entity Server Block". You can connect either a constant block or an event-based random number block to this input port. The duration time in simulation should reflect this input value.
If you have any further question, you can contact MathWorks Technical Support for assistance.