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 file.
We have found one software suitable in our database for this conversion.
Microsoft Windows software - convert dll to icon on Windows
A resource editor for Windows programs
Convert from dll 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.