I currently have a multi dule boot system with multi Ubuntu derivatives. Would like to add Slackware to this setup. Searched the web but found no clear cut method for doing this. Any advice or guidance to a web sight would be greatly appreciated.

What is the procedure you used for setting up your current mutiboot system?

Might be helpful to post which Ubuntu derivatives. It would definitely be helpful to let us know if you are using UEFI or a Legacy install and whether you intend to install Slackware on the same hard drive? Do you currently have the different OS's on the same drive? Different drives? If you are using UEFI, are you planning to use the Slackware Elilo bootloader to boot Slackware from the BIOS firmware option or boot it from one of your other OS's?

Linux kernels and initrds care not what bootloader loads them. Thus, any bootloader can be used for any of them, and consequently, only one Gnu/Linux bootloader per PC is needed.

UEFI makes this even simpler than it used to be, because it has the competence to read files from a FAT filesystem, rather than particular sector groups from odd places.

If you can get Slackware onto a filesystem its kernel supports, if desired, you can get an already present os-prober to add Slackware's kernel (& initrd) to a new Grub EFI menu stanza, or you can compose one yourself, or both.

"Dual" means two. Multi means more than one. If you have more than one OS installed, you have both dual boot, and multiboot. If you have more than two, you only have multiboot.

Thank you all for the in put. Sorry I wasn't more specific so here goes.
I Have a Multi boot system with UEFI as the boot loader. All OS,s are located on one hard drive. A second HD is used to store info. That way files and folders are available to me no matter what OS I boot into. I have MX Linux, Ubuntu various versions (20.04 to 24.04), Linux Light and Zorin 9, All Ubuntu derivatives. I can easily find instructions on the web for installing Slackware but I am unable to find specific instructions for adding Slackware to my existing Multi boot systems. I am using Ubuntu derivatives because they all use the same type of boot loader. So I "think" I know most of the idiosyncrasies. I would like to branch out and try other OS's that are not Ubuntu based. I am having trouble getting Non Ubuntu based OS,s boot loaders to exist within the existing UEFI boot launcher. I Wanted to start with Slackware. There must be something on the web with instructions on how to do this but I have failed in all my searches. Pointing me in the right direction would be greatly appreciated.

Create a partition for Slackware to go in, not an empty space on the drive, along with a swap partition before running setup.

What we don’t know is how you handle the bootloader with each new installation. Slackware uses elilo, but gives you the choice of installing elilo or not. If you desire to keep one of the other installations in charge of the booting, select no to installing elilo, after installation of Slackware boot into the installation controlling the bootloader and run update-grub or create a custom menu for Slackware.

If you want Slackware to control the booting, don’t use elilo, use grub. There are several tutorials on how to install grub during the Slackware installation.

Every distro installer that I know of gives you the option of installing or not installing their bootloader (usually GRUB). If you simply want to add the new distro to an existing multiboot installation, the simplest option is to say no at this stage. Then just mount the EFI partition and reconfigure the bootloader with which you are already familiar so that it will accept the new kernel and initrd.

btw, Slackware requires you to make your own initrd for the generic kernel (which is the recommended one for installed systems) but there is a nice script in /usr/share/mkinitrd that will tell you which modules you need to include with the mkinitrd command. Alternatively you can use the huge kernel with no initrd.