Mac OSX and Windows NTFS are incompatible file systems, so in order to make one work with the other as you describe:
* You must partition the storage medium with two or more partitions and provide for a dual-boot system. -or- * One of the operating systems must provide intrinsic support for the other - an emulation tool, native drivers or provide a shell. -or- * One of the operating systems must provide a tool that permits re-provisioning and co-location.
Since you already have Windows loaded on your machine, re-partitioning would be destructive. Sure, there's dual-boot tools and even some that claim non-destructive re-partitioning, but YMMV and rule #33 of Mike's rules of happy computing is don't trust 3rd party hacks when it comes to your boot sector.
Windows doesn't intrinsically support OSX. OTOH, OSX does have intrinsic support for windows. The problem, as stated, is that the egg came before the chicken in this instance.
There are no tools I know of for Windows that permit re-provisioning the partition/boot sector/OS load and file system to play paddy-cakes with a Mac OS, so your options are hence somewhat limited.
The madman is not the man who has lost his reason; the madman is the man who has lost everything except his reason.
After posting this question, I continued my previously fruitless search and finally came up with this[^] piece of software. I may give it a try after a complete backup. If it works, great! Otherwise... I hope my backups work.
Something I've begun doing for my multi-boot machines is to just put different OS's on different drives.... and choose boot OS by adjusting boot device order. Works well and there's never any interference... and it's really easy to do (provided you can easily change the disk).
Does it happen to have room for two drives? ...if so, you can still do this, just pop one in, load OS, remove, pop the other load OS... place both in, choose OS by changing boot order. One of my laptops has two drives in it but it's not as common on lower end models (or smaller/sleeker models).
Oh well, sometimes you don't think about things like that until you've already purchased the product. Now you know for next time, make sure the hardware is reasonably accessible. I just bought a new desktop and it has easily removable SATA drive bays, so I load different operating systems on different bays. I can easily swap drives when I want and they also have a key lock so I can secure them if need be.