It should be quite possible to convert WBSF backups to a common ISO image using the Wii Backup Fusion software.
Convert Nintendo Wii backup file system file to CD/DVD/HD DVD/Blu-ray disc ISO binary image.
We have found 3 software records in our database eligible for .wbfs to .iso file format conversion.
Microsoft Windows software - convert wbfs to iso on Windows
A CISO to ISO conversion tool
Convert from wbfs file format
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned wbfs to iso. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
A tool for converting Nintendo Wii disk images
Multiplatform software - convert wbfs to iso on any platform
Complete solution for Wii games