Well you can do it in many ways. Do you want to erase one of the systems and install Yoper on that partition?
Anyhow, you need a separate partition for every OS, so if you don't have one free you have to either erase one of the OS-es or
create a new partition by squeezing the others, using a program like PartitionMagic or qtparted (I heard it can do the job,
but never tried it).
And about MBR - sure if you tell Yoper to override MBR, it will do so
It's not a problem though, cuz it should detect your Windows partitions (and I think linux as well). If it doesn't, then
you can manually edit /etc/lilo.conf and rewrite the MBR to include the other OS-se you have.
You can also not overrite MBR - but this way you'll have to figure out a way to start your Yoper system somehow (it won't
appear in the current lilo/grub menu just like that). So either way, so'll probably need to edit your lilo configuration file,
which is easy so if you're careful and think what you're doing you should be fine.