Dear Kenneth,
That is correct. A "buy all/sell all" rate structure is sometimes called a "gross feed-in tariff," and, although it is rare, there are few examples of its use in the United States.
You can model a project that sells all of the system's output to the grid with a production-based incentive (on the Incentives page, under Direct Cash Incentives). Be sure to set the Term for the PBI to the analysis period so that it applies to the full period. You can control whether the payment is taxable or not using the check boxes under Taxable Incentive.
However, SAM assumes that the project only purchases power from the grid for the difference between the load and the system's output, so there is not a way to model the situation where grid purchases cover the entire load.
Best regards,
Paul.