Can I use a BSD kernel in a system currently running Linux?
 

Don't ask why. Just curious.

And if there's some sort of guide on how to do it, you got a link?

Do you mean replacing the Linux kernel with the BSD kernel, while leaving the rest of the system unchanged? If so, the answer is no.

The libraries and applications all use system calls that are specific to the Linux kernel. They would have to be recompiled to run under a BSD kernel (and this is indeed possible with most Free Software applications).

But that said, if you do a fresh install there used to be Debian GNU/kFreeBSD which was the GNU userspace with the FreeBSD kernel. However, the systemd cabal was aiming to cripple that to the point where it went away on its own. It looks like its on the rocks, but would probably accept work if you have the time and the skill and inclination.

https://www.debian.org/ports/kfreebsd-gnu/

You may be able to access data on a Linux system from a BSD system, but that would depend on the Linux file systems supported by your BSD.
(Also, might be possible if your BSD has Linux compatibility.)

The was the Debian effort, there was one from some Arch Linux users some years ago.

But aside from being interesting novelty projects there is little point, which is why none of the attempts have really taken off.

If you read this you will see some of the rationale, but much of it is now irrelevant:

https://wiki.debian.org/Debian_GNU/kFreeBSD_why

The "Why would you prefer Debian GNU/kFreeBSD to Debian GNU/Linux?" could equally be "Why would you prefer FreeBSD to Debian GNU/Linux?"

But also: "Due to license and patent issues, ZFS is unlikely to appear on Linux.". No longer the case.

The "Why would you prefer Debian GNU/kFreeBSD to FreeBSD?" section boils down to ideological reasoning and "If you like the Debian package system"...

I see no benefits with the Debian packaging system of FreeBSD's pkg(7) and the ports tree.

There is also no obvious benefit to taking the FreeBSD kernel and tacking it onto the GNU userland as a replacement to Linux, for a person who is already using FreeBSD.