In order to change Redcore into Gentoo, you need to switch to source mode, using vasile:
In more clear terms, it will transform your Redcore system into pure Gentoo. Binary packages from the repository will be ignored, and you will only install packages building from portage tree using emerge. It will fetch the full portage tree, the overlay and portage configuration files and adjust /etc/portage/make.conf for ebuild only usage.
In this system state you can modify whatever you want in /etc/portage/. You can adjust useflags, keywords, masks, build environment and rebuild the whole system to suit you.
But I would only use this system state if you have a strong knowledge of Gentoo tools e.g.: emerge, equery, layman, eix, qlist, useflags, keywords, masks.