Janet,
Perhaps the business should use the Fiori app. Or, create some nice reporting in a separate BW instance. You have to ask yourself: What benefit would the planners gain by adding this new bit of information to the SNP planning book?
Standard SNP Planning book as supplied by SAP is a planning tool.
It is not a reporting tool
It it not a budgeting tool
It is not an analytics solution.
Adding Non-standard Key Figures to SNP will have an effect on performance.
If you try to torture SNP into being something that it is not, you will add cost (both obvious and also unseen) to the solution. You should only do that if the business requirements are compelling, and the value of the benefit exceeds the costs.
Best Regards,
DB49