Pretty sure you could use this, haven't tested it yet but it should work during the game but i've only ever used the one that you need to set before the prematch countdown as seriousHD- has mentioned in his previous comment.

    setgametypesetting( "scoreLimit", getgametypesetting( "scoreLimit" ) + <VALUE HERE> );

And how do i set it in the prematch countdown ?

you dont need to set it before prematch count down, it will work any time!

setgametypesetting( "scoreLimit", getgametypesetting( "scoreLimit" ) + <VALUE HERE> ); this kicks everyone out of the game