It is not possible to create or edit a group to get the result retroactively after a pulse has closed.
The reason for this is that when a survey closes and the results are generated, it is linked to the specific groups that exist and reflects what the groups looked like at the time the survey closed. On Sunday nights, the system checks all groups and generates results for all groups that "pass" our anonymity checks.
Due to this, if we were to allow the result to change after the survey closed, not only it would generate misleading results that would be hard to keep track over time, but it would also mean that we cannot guarantee anonymity of the respondents, hence it is not possible.