Uhm I don't quite understand what you did and how you came to this, but basically, you have a large file compressed in a multi-volume RAR, but are missing a few parts, correct?
You should know that "CRC32" refers to a 32-bit number which is the result of a certain mathemathical algorythm applied to a sequence of bytes. The same sequence of bytes will always return the same CRC32.
Hence, CRC32 is used as a way to verify for correct extraction of compressed data, trasmission of data, etc. The CRC32 of an uncompressed file is stored along with the compressed file. When you extract the file, the software recalculates the CRC32 from the extracted file, and if it differs from the original, it's a strong indication that the extraction was unsuccessful - typically because of data corruption.
This all means that you don't change a CRC32. It would be like saying "hey, let's change 2+2 so that it sums up to 5". If the extraction of a RAR file complains about CRC32, it means that your compressed files are corrupted (in your case, you're missing parts), and the original file can't be restored.
There are ways to recover from such situations (missing parts), only if the archive was created with a sufficient level of redundancy (I think RAR supports this natively, people please correct me if I'm wrong). But adding redundancy is something you decide when you're compressing; if redundancy wasn't added upon compression, there's nothing you can do to add it at extracting time.
Which means that - if I understood your situation correctly - you're out of luck...