You can also buy springs only from BDS... but they are a good quality spring and may be more $$... but you get what you pay for. I had 5" BDS's and they were great. Those + 1" BL and they cleared 35's w/no cutting involved.
Back to your original Q... yes you can add a leaf to any spring pack, but if you do not know what rate the leaf is it is a best guess scenario. I have nothing against bastard packs, as they can work well, but again, you have to know what you're working with or simply get lucky if they work out.
Back in the day... places like Rancho and RE sold add leafs for extra lift. About all they did was make the whole set up ride like a tank!
'05 LJR, 6sp, RC long arm w/MC 6" coils, 5.13 D44's w/chromo shafts, MetalCloak fenders/sliders/body armor, 37x12.50 MTR's, Warn 9.5, RockHard swing out w/full spare. <><