Create a new position in mainbody_top

Hello,

I want to create a position inner_inset in mainbody_top position. Is this possible ?

I read the tutorial for the creation of new positions, but where to put the code in the file position mainbody_top in default.php file

Thanks.

mainbody_top is a regular position so all that you can do is to create new position inside parent container for mainbody_top. Whole process is described here >> http://www.gavick.com/documentation/joomla-extensions/how-to-add-new-module-position/

Without creating a new posision, is it possible to duplicate innert_inset so she also appears in mainbody_top ?

You can't include one module position inside other one. In Joomla! it is not possible.