You can customise dynamics crm pretty much infinitely.
To the point of completely replacing the standard front end.
You could actually write an asp.net application which used the dynamics crm services and completely replace everything.
Or an installed windows application, for that matter.
So yes, you can do pretty much whatever you like.
Your first choice ought to be just tweaking what you get out the box though.
https://msdn.microsoft.com/en-us/library/gg328027.aspx
https://msdn.microsoft.com/en-us/library/gg328235.aspx
https://www.microsoft.com/en-us/download/details.aspx?id=45022
I would also buy a book or two.
The unleashed and ms dynamics step by step were good for previous versions, I don't see them for 2015.