Both .zl and .zlib files are compressed archives in Zlib format. There is no need to convert them, you can simply rename the extension from zl to zlib should you really need it. Although programs that work with Zlib format should be able to recognize these files regardless of their file extension.
Convert Zlib compressed archive to Zlib archive.
We have found one software suitable in our database for this conversion.
Linux/Unix software - convert zl to zlib on Linux
A data compression software library
Convert from zl 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.