You can probably find programs that offer direct way of converting RAR archives to ISO format. However there is always the option to simply extract the RAR archives using any program that supports them and create a ISO file using some of the disk image making utilities. But nowadays a lot of programs can create ISO images.
Convert WinRAR RAR compressed archive to CD/DVD/HD DVD/Blu-ray disc ISO binary image.
We found one software available in the database for this conversion.
Microsoft Windows software - convert rar to iso on Windows
A CISO to ISO conversion tool
Convert from rar file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.