Given one disk (e.g. /dev/sda) which is known to already contains some partitions (e.g. /dev/sda1, /dev/sda2, /dev/sda3). Is it possible to run preseed on such disk (meaning: partition the rest of disk, set dual boot with grub)? If so how to achieve this?
Yes, it is possible to install a second (or third, or fourth) operating system. However, there are some requirements and caveats that need to be considered.
Depending on what OS(es) are already installed, you may have to use LVM2 (Logical Volume Manager) or similar if you want the new Debian based OS to use more than one partition. In particular you will need to be aware of the restrictions for booting a Linux kernel from certain partition formats (last I knew the GRUB and Kernel files had to reside on a non-LVM partition).
To achieve an automated (preseed, kickstart, etc.) install of a dual-boot configuration you should be very familiar with the bootloader(s) the system will use and how they interact for different OSes. The automated installation could simply fail and do nothing, or it may fail to configure the dual-boot nature while preserving data, or it could fail spectacularly and overwrite/destroy data on the preexisting partitions. Be cautious if you are new to automating an install.
I suggest getting familiar with the official Debian Preseed documentation and the many fine examples they share at that page.
Depending on your disk space; if you have some free space on
/dev/sda, I do not see any contradictions of creating next partition (or partitions even) to achieve dual-boot. If not - you will need to shrink your partitions that might be dangerous, but it is possible to be done as well.