You could Use Git and TFVC repos in the same team project
In Team Foundation Server 2015 Update 1 and above, a project
administrator can add a Git repo to a team project created with Team
Foundation Version Control (TFVC). You can also add a TFVC repo to a
team project created with Git. This allows you to adopt a new version
control system while preserving all the data in your team project.
There should be a local git repo corresponding to your remote github repo or tfs hosted git repo. You could add a remote repo in a selected local git repo from Team Explorer-- Settings -Repository Settings

After binding with two remote git repo one for GitHub, one for TFS; you could simply pull down the changes in GitHub to your local git and then push/sync changes to your remote TFS git repo.