If you use SFM to create Work Order from Case, you can try to use "Source Object Update" in that SFM to update the field you want on your Case (Source) when the Work Order is Created. Update a filed on Case called "Has Work Order" to "True".
On your report question for report type "Cases with/without work orders" I believe you will need to add Cross filter for that b/c you are including by default in your report type Cases that have and do not have Work Orders.
This should get you only Cases that have work orders..
Are you using only SFM to create the Work order from the Case?
If yes when setting up the SFM in the SFM transaction designer you can select the "Source Object Update" and setup for the case to be updated with the information you want upon creation.
Otherwise you can also use the declarative rollup summary app from the appexchange it will enable you to create rollup summary