Follow the steps below to install rEFIt to the EFI partition of an iMac

Download rEFIt and copy the efi folder to the /efi folder as explained below

Before you do the bless command disable the SIP
Boot in recovery mode
# csrutil status
# csrutil disable
# reboot

sudo mkdir /efi

sudo mount -t msdos /dev/disk0s1 /efi

sudo bless --mount /efi --setBoot --file /efi/EFI/refit/refit.efi --labelfile /efi/EFI/refit/refit.vollabel