You can do this in a multitude of ways, I don't see why you need a new parameter.
If you really just want to start a timer in N seconds, what you're really looking for is a timer to start the timer:
.timer 1 5 /timerACTUALTIMER 0 30 /command_here
That will start the actual timer command in 5 seconds. This is really the programmatic translation of what you are trying to do anyway (you want a delay to trigger a delay, and a delay in mIRC is just a /timer, so replace "delay" with "timer"), so it makes sense to do it like this, and should be perfectly intuitive.
The HH:NN:SS format is meant for starting timers at an absolute time of day (something that would otherwise require a constant 1sec timer running to check the time, which is a lot more complex than the above example and somewhat worth having builtin functionality). If you're just looking to start at a relative time, the above example is easy to use and already available. But if you must use the [time] syntax, it's also very easy to convert relative times into absolute times:
/timer $asctime($calc($ctime + 5),HH:nn:ss) 0 30 /command_here
That would trigger in 5 seconds. You could simplify the above with an identifier alias (alias reltime return $asctime($calc($ctime + $1),HH:nn:ss)) and use /timer 1 $reltime(5) 0 30 /command_here.
Given the two options, I really don't see the need for a builtin switch, this is already easy to do.