Add/remove level/duration to potion effect

If I wanted to make a procedure that, whenever it's run, it adds a small piece of time to a potion effect, extending it, how would I do that? Is there a clean way at all? If it runs once, you would have 10 seconds of speed, but if it runs twice, it doesn't reset it to 10, it makes the total 20. Or if I wanted to do the same thing, change a detail about it like the level or remove some time, how can I do that? Is it even possible currently?