how to insert a vlookup into a pivot table - calculated field?

[zazzz]

Please see attached picture.

In this pivot table I've got a bunch of GL #'s. That would be the 6 digit number, 476050 etc.

Besides this number I'd like to have a field which shows what specific GL this number is for. This text would not be part of the source data so I'd have to get it from somewhere else via vlookup. Can anyone tell me how to do this? My boss mentioned its possible and that he thinks a Calculated Field is the answer. I've tried looking that up to no avail.

Thanks!
Dan

PS - I cannot modify the source.

Pivot.PNG

[Neil_]

A calculated field works on data already in the Pivot cache. Add a column to your base table and put the Vlookup there. Include this new column in your Pivot source

[zazzz]

A calculated field works on data already in the Pivot cache. Add a column to your base table and put the Vlookup there. Include this new column in your Pivot source

Oh yes I should've included, that's not an option. The source cannot be modified.
We dont have to go the calculated field route btw. Any route is fine.

[zazzz]

I figured out a workaround. I imported the source data, along with a helper column for GL Desc, into the same workbook as the pivot table. But I still cant figure out how to add the GL Desc here. Can someone advise now? I've been able to use calculated fields for simple arithmetic (to show variances between actuals and budget) but not for vlookups.