You would not want to convert your Windows HDD to dynamic. Doing so will render Windows unbootable.
If you like, since you already have 4 primary partition on the HDD, you could use the tutorial below to shrink a partition (say your D
partition) by how much you would like to use to create the new logical partition with. Partition / Extended : Logical Drives
Of course, adding a second physical HDD to your laptop (if able) would be the best solution for backups. This way if your Windows HDD should ever fail, the second HDD and the backups on it would not be affected.
Hope this helps,