Re: Howto install "18" on RAID0->LVM->ext4->root FS??
If you choose all of the disks making up the md RAID array, in custom partitioning it should see the empty space in the VG, or let you choose existing LVs for reuse. But you're almost certainly better off resizing those LV's, and reformatting them before you get to anaconda. Except /home, obviously, you may want to keep that. For / you will have to reformat it within anaconda.
To use existing LV's, click on them, and then click in the Mount Point field on the right, and type in the mount point. / or /boot or /home.