Since codeplex hub is disappearing, some projects have not been moved by their owners to other places.
What would be the process of moving someone else's project?
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You will need to check the software License. If it is some of the opensource licenses that will enable you to make a fork, is just a matter of importing code on github. Depending on the license, you can relicense, or you will be forced to change the name of your new forked project(maybe someone owns the "brand" of this software, like Linux is a trademark of Linus Torvalds).
Since creating a step-by-step on how to import code on github is off-topic, I suggest you to take a time to read the documentation of the Github Importer Tool. The about page gives more details on what source control software is supported. From here, it is just a matter of set the Codeplex url on your new Github repo and execute the import.
This answer can also apply to other repository management tools, if they use the same Version Control Source tools that are supported by github. You can even use external tools to make the source code migration if using github importer is not an option for any reason.