Nearly everything is possible why not ?
Rename it to StopWatch2 and u have another one

Make the var "running" global. Then u can stop the running StopWatch function by setting running to zero.

var running;
void StopWatch(var* Seconds)
{
static var seconds;
seconds= *Seconds;
while(running)
{
wait(1);
seconds += (timer()/1000000);
}
return;
*Seconds = seconds;
return;

}

This is what I am currently using (as I only need seconds), but it is not working.

Starting it with running=1; StopWatch(SS);
doesn't work...

Here are the worst things:
1. *Seconds = seconds; is dead code.
2. There is no advante in making seconds static.
3. You cannot use timer() over two frames. Read the documentation. Besides timer() is absolutely not necesary here. time_step is totally sufficient.

Could you please explain to me how I could use time_step to make a stopwatch that just counts the seconds?

Nevermind - I've got it. Thx anyway!