You mean for the combat gamemode?

I've long tried to find a way to mod this, but the approach was not trying to look for a way to mod it for the combat gamemode, but to look at other gamemodes. Basically what you want to do is use the capture the flag gamemodes, those have infinite time.

For a better substitute for combat, you want to make a simple mod where you copy paste all the combat gamemodes maps in and then rename the combat map layers to be the capture the flag mode. This way you get the same maps with no capture points. The game will never end as a result on its own (until you decide to stop or the game inevitably crashes).

If you don't know how to set up a mod, I could walk you through it step by step I guess.