First of all, shareaza is not
mldonkey. We believe that the best results are achieved by connecting to a few carefully selected networks. The team has very strict requirements for adding new networks including proper hashing of files, unique content, and that the network code is open source. Also there must be a developer willing to do it. Already this eliminates 99% of them.
Secondly shareaza's networks are tightly integrated with each-other, which is the key to shareaza's streaming of downloads from multiple networks. It is very difficult to properly integrate another network without creating serious bugs and this is not something that can just be plugged in.