To make pour over coffee can I use a cup to pour the water on the grounds? How can I best approximate the proper water temperature? I do have a kettle to boil the water but a cup gives me more control in the process.
There are two important factors here: temperature and control.
Temperature is probably the most important factor here. You don't want the temperature to be too low, when that's the case you won't be able to extract all that you want. You can easily ensure you're not using water that's too cold, either by pouring straight from the kettle (goose-neck or otherwise) or by preheating the pouring vessel (e.g. your pouring cup).
Control is important for pour over brews, but it's not absolutely necessary. You can simply use a regular kettle to pour but you'll have less control over the flow rate (how fast water flows out) and where you irrigate the grounds.
So it's definitely possible to use a (pouring) cup instead of a goose-neck kettle, just make sure it's pre-heated.