I'm new to 3D printing, and Repetier Server. Is it possible to add an external sensor to monitor the filament and automatically pause the printer if the filament runs out? My printer doesn't have the capability of doing it -- I run an Alunar M-505 printer.
External solutions are possible but require programming skills to implement. Simplest solution is that you can send gcode to server with e.g. curl and send "@pause no filament" as command. That would cause the pause you want. So hack a script that monitors your sensor and sends that to the server and you are done. See docs on server homepage how the web API exactly works.