OpenBSD's default bootloader does not support multiboot. Some users install
sysutils/grub but given that you have 5 OS installed you must be using an alternative boot loader.
The command to mount a file system when running NetBSD is
mount(8)
A fstab entry will mount the file system on each boot.
fstab(5)
He is a NetBSD
wiki on Mounting ffs partitions