You can import the previously used .nmind mindmaps to NovaMind software and save it to the newer nm5 or even later format. It is unlikely any 3rd party software would be otherwise capable of this conversion.
Convert NovaMind diagram file to NovaMind mind map.
We found one software available in the database for this conversion.
Multiplatform software - convert nmind to nm5 on any platform
A mind mapping software
Convert from nmind 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.