It seems that at least one DB to ADF conversion is possible and that is for the file types used by ESRI ArcView (ArcGIS). The actual meaning or even if the DB to ADF conversion is really possible could not be verified.
Convert ArcView object database to ArcView ARC/INFO coverage data.
We have found one software suitable in our database for this conversion.
Microsoft Windows software - convert db to adf on Windows
A GIS application used to work with geographic data
Convert from db 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.