Erm.. I mean technically you can, it's protected, but I don't know of the ramifications of changing it. But in code it's just multiplying tpf by 1.0 - so you could super easily reproduce this yourself without playing around with things I've never even heard of, are undocumented, and probably intentionally written that way as to avoid people doing this. My thinking is that if this was a thing, it would have a public method exposing it with said missing documentation.
tldr; If you want to adjust your game speed, multiply tpf by a speed float. 1.0f being "normal", 2.0f being 2x (fast-forward) and 0.5f being half speed (slow-mo).