Open source software allows you to modify the code for your own uses. I use so many different open source tools that I haven't become a regular contributor to any one project. The regular contributors are amazing, I don't know where they find the time.
When you fix, modify or improve an open source software project, even if you don't become a regular contributor, send your changes to the project manager. People have been very good at including changes I have sent in.
This will help you in the long run. If you change your code and don't send it back the project will release a new version that doesn't have your changes. If you upgrade to the new version you won't have your changes and will have to make the change over again. Eventually you will end up with a forked older version and cannot benefit from the improvements in the new versions. For an organization this can turn into a really large problem when they hire new computer programmers that are used to the newer versions of the software. When one piece of software depends on another you may end up not being to upgrade anything or start using new software that uses a newer version of the software you forked.
So send your changes in, even if you only a change an open source project once.