You are welcome to change ParaView 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 ParaView and having it tested and, eventually, merged into ParaView are located here.
Because ParaView is large and well-established, all code committed to ParaView has to conform to particular standards. In particular, new code must use or be compatible with ParaView’s license. All code will be rigorously tested on many platforms before it can be merged into the mainstream.