For running a command on point capture you can do:
PHP Code:
public OnPluginStart()
{
HookEvent("teamplay_point_captured", TeamplayPointCaptured);
}
public TeamplayPointCaptured(Handle event, char[] name, bool dontBroadcast)
{
ServerCommand("sm_addtime 600")
}
As far as limiting the command to only run once per point I'm not sure too sure. Please share if you end up finding the solution.