Hi yikenW
I am Dave, an Independent Advisor, I will help you with this . . .
It is not possible to make Windows 10 think that an external drive is internal and also, if that were possible, it would not be possible to make Windows 10 extend the C drive into that space, what you can do is permanently connect that drive to yoru system and move some data to that drive
1.
Move all your User Library folders to D:
Open Windows File Explorer
In the right hand pane, right click your Documents folder and choose Properties
Select the Location tab
In the Location box, type D:\Documents then click Apply and Ok
When that move completes, do the same for Pictures, Videos and Music
Each time, provide a new folder location, for example, Pictures will go to D:\Pictures
2.
Move the location of all future content downloaded from the Microsoft Store
Open the Settings App (gear icon on your Start Menu)
Go to System - Storage - Change where new content is saved:
Change all options you find there to D
Also, for any Apps or games that you may have downloaded, go to Settings - Apps, then select that game or App and there may be a Move option . .
3.
If you are a Steam Gamer
Stop all processes associated with Steam
Move the Steam folder to your D drive manually
Then open the Steam Control Panel and point that to the new location
4.
Please Note: this should ideally be done before you begin installing any software
For 3rd party applications, some will allow you to choose an installation location, for those, create a new Program files folder on your D drive, then for each application, create a new sub-folder of that folder
Other 3rd party applications will not allow you to change the installation location, and there is no safe way to move those to your D drive . . .