SuperLemmini's External Files system appears to be more versatile than its internal .ini file system. That is, it is capable of loading .lvl files as well as .ini, even if these levels have been created and saved in later versions (than 1.43) of the NeoLemmix editor (such as 10.12).
Since SL generates a .ini file of the currently loaded data, effectively acting as a converter, the resulting level is saved as a .ini file in levels/$external. This means that any level in .lvl format can be converted to .ini via this process. Thanks to Turrican for supplying this information.
If SuperLemmini can in fact load these levels, then could it also generate a .ini file of the currently loaded data, effectively acting as a converter? It appears to do this anyway as part of the process of loading these levels, so can the resulting level be saved as a file?
This topic was originally a question topic which has now been modified to supply the info to others who may benefit from knowing about this feature.