Some special characters, which are not provided by typical keyboard layout, are often uset at my website. I would like to add several buttons to the XOOPS editor, which would insert these characters. May I ask for advice how to do it?

Actually I want to have buttons for characters
ã, ë, é, ò, ô, ù - small and capital.

BTW. XOOPS tags like [url], [img] etc. are added at the end of the text, not at cursor's position (XOOPS 2.0.6). Is there a way to correct this problem?

You should have a look at the following hack listed here.

This adds a row of language specific keys, and can easily modified to your characters I think.

Excelent! Here I made a Kashubian version: