starting from scratch? yes. starting with a pre-existing platform? yes but it's even harder. yes, you have the pre-existing code to 'borrow' from and re-skin and whatnot, but anything new you are adding has to interact with all of the pre-existing code/architecture smoothly. you might go to make one seemingly minor tweak to something but the waterfall effect can totally break the whole system down.
Probably, considering it's like an actual job