Actually, if you have a keyboard that has cursor keys (arrows) on it, you already have mouse emulation. Of course, that is mostly external Bluetooth keyboards. Though I suppose I should note that there are actual Bluetooth mice as well, though I've never used one. (Can't even say I've physically seen one, but a couple of the manuals for tablets I've seen said there weresuch things).
What you're requesting sounds more like emulating one of those trackpads/touchpads found on almost all laptops. Did you check in Play store to see if there is such a thing?