The APIs of "Import build/release definition json files" are the same as Create a build definition and Create a release definition.
Follow these steps
Step 1- get a build/release definition
Step 2- change the build definition name and create/import it to TFS.
For dropping then on a location in source control, you import the JSON file, you will get a new build definition, not a version-controlled file. In future, you may use YAML builds which will configure your CI process as code, but currently, YAML build only exists in VSTS, not on-premises TFS.