Can an admin override a step that has been denied?

There is not an option to override a step that has been denied, but you could choose to add in a denial step within the workflow to provide an administrator an opportunity to review a proposal that has received a denial.

You can add a denial step to any approval step by adjusting the section under 'What happens if the proposal is denied' and setting it to 'Custom Step.' This would allow you to provide an additional approval step that you can then include the system administrator on, and you can determine if the workflow should continue on from this point or stop.

There is more information about denial branches and how to set them up here.