I use R.W. Knudsen quite often and have had no issue getting it to ferment. Even if sorbate has been added it is typically at a minimal level and should not interfere with fermentability.
I would use this recipe:
128 oz Welch's White Grape Juice
4 frozen WWGJ concentrates, thawed
96 oz R.W. Knudsen Cranberry Pom juice
water to 3 gallon mark
1 1/2 tsp. pectic enzyme
3 tsp. yeast nutrient
Invert Sugar Syrup to reach desired SG (no higher than 1.085)
Adjust acid, if needed (though this must will be acid just from the CranPom--you likely will not have to adjust)
3/8 tsp. tannin
Lavlin 71B-1112 or K1-V1116
All "grocery" ingredients should be at room temperature. Prepare your must by combining all juice and water in a fermentation bucket...juice/water should just hit the 3 gallon mark. Check SG. Adjust starting SG by adding invert sugar until "target" is reached. Now add the tannin, pectic enzyme, and yeast nutrient. Cover primary and set aside for at least 12 hours. Add activated yeast (follow package instructions) and cover primary with light towel--secure cover to keep any foreign matter out, but make sure oxygen can get in. Ideal temperature 70-75F, but consult yeast label if you have concerns. Be sure to stir twice a day & check SG daily. When SG has decreased by 2/3 transfer to carboy and attach airlock. When wine is no longer fermenting (SG checked for 3 consecutive days with no change) rack off of any sediment and add k-meta for a 3 gallon batch. Keep under airlock until ready to bottle. Note: the lees should be fine because you are working with juice. Rack off of sediment approximately every 60 days until wine is crystal clear and no longer dropping any sediment. Proceed with back-sweetening and stabilizing, if so indicated. Rack one final time at least 7-10 days after stabilizing/back-sweetening and bottle when you are ready. Serve chilled and enjoy.
*** Keep us posted as to how this turns out for you and what recipe you do use. Enjoy!