There appear to be 2 ways to install extensions? One is to install the manager. Does the manager have a built in .7z extractor? I'm not keen on downloading yet another extraction program. PKZIP is what I'll stay with til I die.
The KEM manager from kmext.sf.net containes a 7z.dll plugin, and a 7z.exe
So yes, it is automatically included It lands in the directory "tools".
If I want to find out such things before installing, I just click on the xxx.7z link, and instead of "save" choose "open", that shows a list of all the contained files and their folder structure.