You cannot convert just any random dynamic library (.dll) to icon (.ico, .icon). However, some .dll files contain icons that can be extracted and saved in either icon format or common graphics format with some suitable tool, such as IconExtract or ResEdit.
Convert Dynamic Link Library to Icon image.
We found 2 software records in the database related to .dll to .ico file format conversion.
Microsoft Windows software - convert dll to ico on Windows
A resource editor for Windows programs
Convert from dll file format
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned dll to ico. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
A small utility for extracting icons