Never ever add a symbol to a system library. An update to kicad might delete your precious symbol.
Always work on your own personal libs.
Yes of course. We are always happy to receive a new contributor.
(Bellow i'm assuming you know how git works. If not i can give you more detailed instructions.)
First read the kicad library convention maybe also have a quick look at the FAQ. Only contributions that comply with this rules can be accepted. (If there is a good reason why your contribution breaks a specific rule just state it in the pull request. It will most likely be accepted this way.) We want a lib of good and consistent quality.
If you are finished with that you can start with the real work.
First create a github account and fork the library repo
Clone your personal fork to your local machine and checkout a new branch.
Do your changes and push them back online.
Go to the github page of the library repo and open your pull request.
One of the librarians will look at your contribution. In most cases there are some minor adjustments to be made. (Especially in a first time contribution.)
If the library manager is happy with your contribution it will be merged into the repo.
But your local kicad installation will not automatically get these new symbols you shared. Symbols and 3d models are not updated. (They come with the kicad version.)
So you will need to use local personal libs for this. (You could update the libs by hand but this way you can not easily share your schematic with someone else.)