You have to upload all your input RAR files to this website and it will convert them all to ISO format. Besides ISO, it also lets you convert RAR to other formats such as 7Z, CAB, ZIP, TAR, JAR, etc.

If you have an already compressed (.rar) file, you can split/convert it into multivolumes through the WinRAR UI. Open WinRAR and navigate to the folder that contains the RAR file.