Except a script or app I should not know .... I don't know any possibility to set a definite pitstop time in AC servers.
I suspect you'd wish to avoid cars pitting only for 1L refuel ... if it's the case, just increase the fuel use for the cars ... so that it's necessary for drivers to refuel more and spend more time in pitstop.
Easy if they're driving the same car ... a bit more preparation with different cars.
Thank you for your answers ^^ I am hosting a server for a GT-League and in the real rules there is a mandatory pit time (65 seconds you have to stop). Thats the reason i asked.