ISO-8859-1 is specified by the GZIP format. It should never be Encodings.Default, or you will generate a GZIP stream that is not compatible with any other GZIP library or tool. It could be that you don't use the GZIP function in the library at all, in which
case it does not matter.
IBM437 is specified as the default encoding for zip files by PKWARE in the ZIP specification. UTF-8 is also specified in the ZIP specification.
Changing any of these encodings to Encodings.Default in the source code will produce a library that does not comply with the specifications. It may produce zipfiles that are unreadable by other tools.
I think that you want to be able to produce and read zipfiles with entries that have filenames containing Simplified Chinese characters.
If this is true, you can accomplish what you want through the documented, public interface of DotNetZip, without modifying the library source code.
If you want to create zip files that contain entries with filenames using a particular encoding, there are ctor's for ZipFile that accept an Encoding. You can pass Encodings.Default there. This will produce a zipfile that uses the Simplified Chinese code
page, and your filenames will be properly stored. You must be careful to unpack such a zipfile using the same code page. Check the doc on DotNetZip for more information on that.
You can also set the ZipFile.UseUnicodeAsNecessary property, and it will produce zipfiles encoded in UTF-8, which will also allow Chinese characters. However, as I described in the doc, a UTF-8 encoded zip file, while it is correct, may not be compatible
with Windows Explorer.
The published interface that supports Unicode is sufficient to allow programs to create and read zipfiles that have entries with filenames containing Simplified Chinese characters, or characters from any code page. You do not need to change the library
source code to handle Simplified Chinese.
I hope this helps you out!