Hey Carlos,
Sorry to say but the z-index option isn’t available via BB. We like to keep things simple and it feels like the z-index option is rarely being used. You can achieve what you want though if you check the KB article below.
http://forum.wpbeaverbuilder.com/knowledge-base/transparent-headermenu/
You can set the Header, Content, and Footer backgrounds to transparent. Simply go to their settings under the Theme Customizer and delete the default hex value then click out of the color picker. That should make the selection transparent.
Regarding the Fixed Header, if you’re trying to achieve that layout, it requires custom CSS. I’d suggest applying custom CSS to set the colors to white on the transparent header then make the default link color to black. But you can head over to our User Voice Forums if you want to add that as a feature request. 
Hope this helps!
Ben