If I want to show that a problem is np-hard is it ok to use a existing np-hard problem multiple times? For example use Hamiltonian Cycle n times in a graph where n is the number of vertices? Or do I need to transform the graph into something that can easily be solved by an existing np-hard problem used 1 time?
You need to show the exact oposite.
It doesn't prove anything if you prove you can solve your problem with an NP-Hard problem. [You can solve every problem in NP using SAT, by Cook-Levin Theorem].
You need to show that if your problem is solvable in polynomial time - so is an NP-Hard problem. That what a reduction actually does.
For example: If I can show I can solve shortest path, using TSP - does it make shortest path NP-Hard? Of course not! It only shows TSP is at least as hard as shortest path!