Hey there!
The code of the kit is entirely open so you can go ahead and make as many additions to the functions as you need. Unity generally doesn't limit you with that. You, of course, need to know how to write code in C# :) The same goes for all assets. You can simply import your own sprites and replace the included sprites with them. If you haven't already, I would recommend joining the discord server. You can always ask for help there and other users also share their creations. So you will be able to see how they used their own assets.