Any reason you need the data in a spreasheet? (IE do you need multiple related worksheets in workbook etc), if not and you just want a simple capacity report from WFA on your aggregates or volumes then it is "possible" to have WFA invoke a SQL query for your volume\aggregate capacity, output it as a .csv file then add a WFA return paramater for the URL to download the .csv file (or have WFA email you the .csv file to you). This can be used as an alternate method instead of creating a custom OCUM report...however "possible" does not necessarily equate to "advisable"
There are multiple ways to get this information you are probably looking for however considering the WFA database is just a cache of the OCUM database have you considered using the OCUM to schedule a report and email it to you in .csv or .pdf format??? I'd advise looking at the default "Aggregate Capacity and Utilization" or "Volume Capacity and Utilization" reports.
You "could" re-create the OCUM reports using WFA or a script...but why re-invent the wheel?
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