What you're looking to do is convert a physical machine (aka, your Windows 10 partition) to a VirtualBox VM. Sorry to repeat the task ahead of you, but using these terms I've come across this helpful answer courtesy of Chris Hoffman of HowToGeek.
For VirtualBox – A Manual Method
VirtualBox doesn’t offer an easy graphical utility for converting a
physical machine to a virtual machine. If you’re feeling particularly
adventerous, they do offer some unsupported instructions for
converting a physical Windows computer to a VirtualBox virtual
machine. This requires some registry tweaking and shutting down the
computer. You’ll then have to manually create a copy of the disk and
convert it to a VirtualBox VDI file. The process for converting a
Linux virtual machine should be about the same, but without all the
extra tweaks required to make Windows behave. It’s much easier to move
Linux installations between different machines with different hardware
This process isn’t for the faint of heart, and we include it here only
for the sake of completeness. Even if you use VirtualBox already, you
may want to just use VMware or Microsoft’s utility and give VMware or
Hyper-V a try. If you’re still ready to give it a try, consult the
MIgrate Windows page on VirtualBox’s wiki.
My personal belief, when faced with the above, is that imaging the existing Windows 10 partition/installation to VHD would be easiest and smoothest option available, as VirtualBox is 100% compatible with this format/standard, although of course this incurs the needed dataspace in which to store the exported image.