I've never had much luck with the wash-in brands, so stopped using them.
Recently relented and tried the Polartek wash-in for softshell trekking pants and a hardshell rain top, but won't know until a heavy rain experience next spring if it works - and it is a restore product not an add-on.
The one time I was in your situation, it was an MYOG pack with a ripstop from OWF called 'storm' that was very waterproof, but discovered after making the pack that the uncoated side had no DWR. Used Tectron polymer (not silicone) fabric spray with great success - the DWR remans good to this day. BUT, it was the old Tectron formula.
Haven't found the new one to be as good. Another polymer spray worth considering is ScotchGuard "heavy duty water repellent for outdoor fabrics".
Hope others will add to this with their experience with the sprays.