How can I extend system partition when there is no unallocated space on my disk?
I agree with Jan, shrink one of the volumes and use the space freed up to create the new partition. Like Jan, I like to use EaseUS Partition Manager although there are many other partition managers (e.g. Partition Magic, GParted and various commercial ones from Acronis, Paragon Software etc) which are also excellent.
Do you have another partition on the hard disk containing the system partition? Technically you cannot extend a the system partition across multiple hard disks. This is only possible if the underlying disk actually is a RAID volume
If you do have another partition you first have to shrink it so there is free/unallocated space next to the system partition. Then you can use the tool linked by Hovsep or if you prefer the Easeus Partition Master to extend it.
Best Tips for Adding Unallocated Space to System Partition Windows 7 You Can't Miss
How to extend system partition when there is no unallocated space next to it?