1 - Linux compatibility
#FreeBSD actually has a Linux compatibility layer which allows it to run #Linux (ELF) binaries without a problem. Keep in mind though that this isn't full proof, but many things can be made to work without issues. You're going to need 2 things: support in the kernel and a userland to provide optionally required libraries (the latter isn't needed for the #jail but I'd still install it anyway in case you're also going to use this setup for more purposes).

https://forums.freebsd.org/threads/setting-up-a-debian-linux-jail-on-freebsd.68434/

Setting up a (Debian) Linux jail on FreeBSD

Hi gang! Prerequisites: I am assuming that you know how to install software on FreeBSD and also have some basic understanding about FreeBSD jails. Editorial I'm going to be very honest here: I...

^{ShelLuser (The FreeBSD Forums)}