Emilie,
Since you are mentioning wanting to use a SAP Connector and import, I think the easiest method would be a combination of things suggested above, with some additional features.
I would have the import step show each month, with the Optional Data Load set to true. Then you could create a business rule/function that auto completes and locks those steps for months 1-11, preventing input to them, leaving only month 12 open for input. This would be the least amount of changes to your current setup.
Also, if you dont want to spend time on developing a function to perform the locking of these steps in 1-11 its a quick manual task that can be carried out as each new year starts using the Workflow Batch processing.
