I would like to know if it's possible to use one XOOPS installation for more than one website. I read the manuals but could not find an answer.

Currently, it is not possible "out of the box".

However, there is a development of a "Multisite" module by Wishcraft.

Be aware that this is a BETA version.

This Q&A was found on XOOPS Web Application System : https://xoops.org/modules/smartfaq/faq.php?faqid=722