How can I shrink my Mac HDD and add the space to my Windows 10 disk?
There are both paid and free tools capable of doing that. Here are some pages that may be helpful about your choice of software and its use:

http://guides.macrumors.com/Extend/Resize_Boot_Camp_Partition
https://superuser.com/questions/451192/resizing-bootcamp-partition-without-wiping
https://discussions.apple.com/thread/2027812?start=0

Generally speaking, the steps you need to follow :

Backup both Mac OS and Windows
Install rEFIt or burn it to a CD
Resize Mac partition inside Mac OS
Boot recovery media (gparted or paragon live-cd etc.) and resize Windows partition
Boot to rEFIt and sync MBR and GPT records. Or alternatively you can disable SIP and use the gdisk command line tool as explained here to recreate the Hybrid MBR.
Boot to installation disc/USB of the Windows version you have and repair Windows installation. If the automated “Start-up Repair” option doesn’t work, you can try running the commands documented here.

There may be additional steps depending on the software you use.