I'm setting up some domains and subdomains on my basic hosting plan and I'm wondering if it is possible to point an adondomain or a subdomain to a subfolder of a second level (root_folder/subfolder/folder_to_which_subdomain_points)?

I've heard it could be possible by .htaccess, but have no idea how. Does anybody know it.

Don't try using the cPanel Redirect function to do this, as it will trip up on subdomains and add-on domains. You'll have to manually edit your .htaccess files (one in each subdomain or add-on domain root directory).

RewriteEngine On
RewriteCond  %{REQUEST_URI}  !^/second_level
RewriteRule  ^(.*)$  /second_level/$1  [L]

Something like that (I'm working off the top of my head on about 3 hours sleep). The idea is, once control is in the root directory of the subdomain or add-on domain, you exclude cases where you're already starting at the second level, and redirect to second level+original request. It may require some tweaking, but it should be close to that. If you want the visitor to see the new address in their browser, replace [L] with [R=301,L].