You are welcome to change VTK and are invited to contribute your improvements back for the benefit of the community.
Step-by-step instructions for the mechanics of submitting work for inclusion into VTK and having it tested and, eventually, merged into VTK are located at https://gitlab.kitware.com/vtk/vtk/blob/master/Documentation/dev/git/develop.md.
Because VTK is large and well-established, all code committed to VTK has to conform to agreed upon standards. In particular, new code must use or be compatible with VTK’s license. All code will be rigorously tested on many platforms before it can be merged into the mainstream. Besides the contributions process, you will find extended details about coding conventions, regression tests, bug tracking, and the release process in the The VTK Software Process.