Of course you can install plugins and themes like you do it normally. If it is a prepared MoinMoin package file, then you just need to upload it and click install on the page which lists all attachments. Otherwise you have to copy the plugin file into the directory wiki/data/plugin/ followed by the type of the plugin. E.g. for a theme you would copy the .py file into wiki/data/plugin/theme directory.
You will need to copy the css and img folders (if you are installing a theme) into a subfolder, which has got the name of your theme, of wiki/htdocs.