Program like ISOBuster support great many various disk image file types, including .mds files from Alcohol 120% and somewhat old generic .aa disk images. So it should be no problem converting one to another, but nowadays you would be better off in convertion to something more common, like ISO.
Convert Alcohol 120% Image file (media descriptor file) format to Generic multi-volume CD image file.
We found one software available in the database for this conversion.
Microsoft Windows software - convert mds to aa on Windows
A highly specialized and easy to use CD, DVD and blu-ray data recovery tool
Convert from mds 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.