First of all, look at the class you want to extend, i.e. PurchPackingSlipDP. Find the logic you're interested in. You'll find that the table buffer for lines is purchPackingSlipTmp and its fields are populated in initializeProductReceiptLine(), therefore that's the method you need to extend.
Then look at how the fields are populated. You should soon realize that this report is based on purchase order data - it's based on packing slip journal.
For example, this is how quantity is copied from the journal line:
purchPackingSlipTmp.Qty = _receiptLineSelection.parmVendPackingSlipTrans().Qty;
You need to decide whether you want to add Requester to VendPackingSlipTrans, or you'll try to fetch PurchLine from VendPackingSlipTrans and take the requester from there.
Assuming your want the latter, you just need to call purchLine() method of VendPackingSlipTrans and then access the field. For example:
purchPackingSlipTmp.XyzRequester = _receiptLineSelection.parmVendPackingSlipTrans().purchLine().Requester;
