I need to pull old purchase data with GRN details for reporting. How can I get historical data with additional fields?

To see purchase data along with GRN then you need to add Custom Column and you can follow some steps:
 Step 1: Go to the Display and click on Accounts Books.
 Step 2: Now click on Accounts Register Standard and then select purchase Register and select the option for purchase type and parties for which report will be generate.
 Step 3:Now specify time period and then enable Tax bifurcation and other option as per your requirement for report and then press OK Option.
 Step 4: Open your report and then click on Custom Column option. 
 Step 5: Now enter any name in format name field and press Enter key. 
 Step 6: Now click on Add column option then Specify the heading of the column,Show after which Column,Data type as Text,Alighment,No. of characters,Category in Data field as Inventory voucher body then click on Field. 
 Step 4: Now Select the field for GRN Wherever you have entered during voucher entry wheather it is in GR RR No. Field or add optional field and save the format and then reload report from starting with select Add custom column format name during select time period for report.