Question: I have a Pivot table in excel that is using a raw table as its data source. This pivot table is doing a bunch of grouping and summing of rows. 

I'd like to now use the result of this new pivot table as the data source for a new pivot table which will further modify this data. 

Is this possible with excel? I suppose you could call it 'nested pivot tables'

Answer: 
In a new sheet (where you want to create a new pivot table) press the key combination (Alt+D+P). In the list of data source options choose "Microsoft Excel list of database". Click Next and select the pivot table that you want to use as a source (select starting with the actual headers of the fields). I assume that this range is rather static and if you refresh the source pivot and it changes it's size you would have to re-size the range as well. Hope this helps.
As suggested you can change the pivot table content and paste as values.
But if you want to change the values dynamically the easiest way I found is
Go To `Insert->create pivot table`
Now in the dialog box in the input data field select the cells of your previous pivot table.