I believe what Travis is saying is that provided your existing fire sprinkler system is the same material as the extension, and the occupancy of the areas is the same, and the existing system was properly calculated originally, then using "equivalency" you may be able to size the new fire sprinkler piping equal to the existing pipe IDs. To try that the new pipe would need to be the same distance from, or closer to, the water source as the existing pipe and supplied by a pipe of the same ID as was originally calculated. All that being said, as a contractor that was personally licensed in 4 states, I recommend consulting with an engineer or contractor in your area that may assist you. I own three calculation software programs, any of them would be capable of calculating that work, and one of them only costs a few hundred dollars online and I have compared its output to my $1K calc software and not found a difference in any output. I am not aware that the pipe schedule system referenced in NFPA 13 may not be used for CPVC pipe, the schedule system is a compilation of pipe IDs and the number of open outlets an ID is capable of supplying, so unless the standard states steel pipe only then it is based upon pipe ID and not product. In fact, when I took my first course in hydraulics the instructors informed us that to establish the densities in NFPA 13, that pipe schedule systems were calculated, and the average density that the schedule system provided became the basis of the fire sprinkler design curves.