Is this possible, at all? Like, from my very, very limited experience, W10 specifically will surely allow you to kick it - but afterwards, you have anarchy. Please help me out! 
Hi @Nachtfalke,
To be totally honest, that’s not recommended if even possible.
Mainly because Windows mostly uses NTFS, developed by Microsoft. And NTFS isn’t POSIX compliant, so AFAIK Linux can’t boot of it.
@Aragorn can possibly tell you more.
GOOD ONE!
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We don’t deal with 5 year old users - they aren’t allowed on the forum.
Answering your topic title question
And no - that is not possible.
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You can’t: calamares – Manjaro’s installer – always formats the partition it will install Manjaro on.
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What is the problem with Formatting though? Why are you unwilling/unable to Format accordingly?
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Just convenience, to be honest. Installed Manjaro on my old Dell Vostro, and with the two harddrives (of which one was exclusively OS) this was a twenty-minute affair. Working from that computer right now - state of the art working environment, and about 50% less energy use than with W10. Love it!
Now, today, first day of an event-free, if not work-free week, I’m trying to install Manjaro on an Acer Nitro 5 - one big HD, both for OS and data, and it’s hell. Started the installation process around 1700. Right now, it’s around 1930 my time, and the installation still doesn’t recognize the partition when I’m booting from USB. 
“I shall overcome”, but, ugggh.
is secure boot disabled, is fastboot disabled, is AHCI enabled?
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Hijacking my own thread, but - yeah, currently in the process of trying this:
Seems to have been a problem that others have mentioned here a couple of times, already. The AHCI change is the thing that doesn’t work as smoothly as the rest. - The solution in that thread, about pressing CTRL+S while in BIOS, I cannot confirm so far.