Sure, you can provide access to credentials (such as github tokens) using secrets [1]. Add a secret to your repository and then add a command to your pipeline to write the secret to disk [2]
Alternatively if you want a global variable that is available to all pipelines automatically, you can configure global environment variables at the runner level. [3]
[1] https://docs.drone.io/secret/repository/
[2] http://discuss.harness.io/t/how-can-i-set-host-ssh-key/4636/2
[3] https://docs.drone.io/runner/ssh/configuration/reference/drone-runner-environ/