It seems that .des files (Corel Designer technical illustrations) can only be created in Corel software, so a tiff to des conversion may possible be attempted using CorelDraw. Unfortunately, we do not know about any other way how to achieve tiff to des conversion at this time.
Convert Aldus Tagged Image File Format (TIFF) bitmap image to Corel Designer technical illustration.
We found one software available in the database for this conversion.
Microsoft Windows software - convert tiff to des on Windows
A set of applications used by professional graphic designers
Convert from tiff 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.