Hello all,

Is there a way to install another webapp (a wiki for example) to a subdirectory of a slave like slave_example.com/wiki

If your slave_example.com domain has a specific "document root" on your server, you can add any subdirectories inside this "document root" and therefore install whatever you want.

If the "wiki" webapp that you want to install is a joomla component then you have to install the component on the master website before re-install it into the slave site.
You need to install the extension in the master to have the php code installed and available from the slave. After that, you can create a menu item with the alias "wiki" to reply at url slave_example.com/wiki