It seems that both MELL and MELLEL are documents used in Redlex Mellel, albeit in different version. It may be possible that the latest versions of the program are still capable of producing the older .mell documents for backward compatibility purposes. At any rate, using the original program seems to be the only option here as no 3rd party software supports Mellel formats.
Convert RedleX Mellel document file to RedleX Mellel document file.
We have found one software suitable in our database for this conversion.
Apple macOS / Mac OS X software - convert mellel to mell on OS X
An advanced word processing program for Mac
Convert from mellel 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.