Hi:
Although PROC REPORT could generate your report, when ODS creates a result file -- it creates the WHOLE file -- so using ODS and PROC REPORT, you could not "add" just a column for February to an existing Excel sheet that already contains data for January.
So using the ODS approach, in January, you would create a report that showed only January; then in February, you would overwrite the Excel file with a report that showed January and February; then in March, you would create a report that showed January, February and March. However, the report would not be "additive" -- it would be recreated in its entirety every month. If this approach is not what you had in mind, then Scott's suggestion to look at DATA step techniques is the way to go.
cynthia