You just need to reference the Redhat partition in grub.conf or menu.lst.
The entry is something like:
title "whatever you want to have appear on the menu"
root "where grub finds /boot/grub, so make it the same as the mandrake entry"
kernel "where grub can find the Redhat kernel. Paste in the line from the redhat version of grub.conf"
I seem to recall that Redhat might have had an additional line for the kernel info, but I don't have that with Slackware. If there is another line, just paste it into the new grub.conf/menu.lst.
I did this when migrating my box from Redhat to Slackware, so it should work. In my case, I added Slack to Redhat's grub, rebooted to Slack, re-installed grub from Slack and changed the MBR, added Redhat to Slack's grub setup, cleanup my files etc. and then removed Redhat and its grub entries.
Hope this helps...