It appears that both .dpl and .ddl files are used by Ovation Pro desktop publishing program and some sort of conversion or export may be possible with the original tool. Sadly, we could not reliably verify this information and thus the viability of dpd to ddl conversion remains uncertain.
Convert Ovation Pro document to Ovation Pro DDL file.
We found one software available in the database for this conversion.
Microsoft Windows software - convert dpd to ddl on Windows
A desktop publishing program for Windows
Convert from dpd 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.