If any program can somehow convert or transfer .dnl databases to .ivt databases, it would be Concordance from which both of these file types originate. Sadly, we haven't found any specific information about this file conversion on internet, so if you know any relevant information, please leave notice in the discussion below.
Convert Concordance database to Concordance inverted full text database file.
We found one software available in the database for this conversion.
Microsoft Windows software - convert dnl to ivt on Windows
A litigation Support Software from LexisNexis
Convert from dnl 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.