I have what I think is a suitable workaround, but it requires a bit of time.
GBee what you think: Go to Control Panel > System and Security > Storage Spaces
Here you can form a virtual merged drive between your super fast SSD and that old clunker HDD (or SSHD), but there are some disadvantages and advantages to consider.
1) First you have to be able to offload a backup copy of everything on those drives to someplace secure (like a $100 5TB external drive in my case)
2) That takes a lot of TIME - I did it overnight.
3) Now you have to get rid of everything on your SSD - I did a windows restore, chose to delete all my files (since I had them on an internal SSHD and on that external as a backup)
4) IMPORTANT: When you do that windows re-install, don't include the drive where you saved your files!!! (I even disconnected that external HDD incase I screwed up here.)
5) Now you can create a new pool and storage space which will delete all data from the HDD you are adding to boost your SSD's space.
6) You might lose the ability to insure what goes on the SSD (like gamers who want to run really fast might wish) But Windows operating files should all be there.
7) You might be able to install all future apps to the HDD by selecting a folder that is physically located there.
8) Why bother with all this? There's another solution that might work very well for you. Have you considered the fact that a new windows install only takes up 19.7GB? (That's with the HP bloatware on my SSD). Immediately after you re-install windows do this: Settings > Storage > Change Where New Content is Saved
under each type of new content is a drop down box. Just make all those point to your HDD. It may or may not redirect your Office 365 new install (hope you kept that product key!). Either way, there's room for 1-3GB of office 365 on the SSD. Especially since all your new files are on the HDD.
I'm sure there are other drawbacks to making the shared storage space that I didn't mention, but that solution and #8 above sure seem a lot easier to me than having to edit the registry, especially knowing that the next office 365 update could break your registry 'fix'.