I'm thinking of porting a few assets from Skyrim to Fallout 4 as my first mod for Fallout 4, when the tools come out. Though, I'm curious about the legal ramifications of releasing said content to the public. I've done my research on the topic, but I'm finding people saying different things about the legality of doing (from "nah you're fine" to "100% illegal no exceptions") this but I'm going to focus on ZeniMax's terms of service on using their content.

The terms of service says it cannot be done commercially:

"Commercially" is defined as:

A friend says that means I can only upload it to a website that has no ads, donation button, or anything of the sort. Would the Steam Workshop fit that description?

No, it is not. If it is derived from Skyrim game assets, it cannot be ported unless said derivations are excised. Same goes for any previous BGS game.

We've been working under such a setup since Morrowind-to-Oblivion. We veteran modders are very familiar with the rules of porting assets.

what do you want to export exactly?

The only thing that matters is the first line of the passage you quoted: "You may not copy or download any Content from a Service unless You are expressly authorized to do so by ZeniMax in writing."

Get written permission from ZeniMax and you're good. However, they have given a sort of blanket statement of "don't do it."

You can do it for yourself but not distribute it. If its content made by a mod maker your porting, you may want to check their policy on using their work first.

You are not allowed to port anything from one Bethesda game to another and release it publicly, no matter if the website has adds or not. They can't really shut down anything you keep privately, but I wouldn't go bragging about breaking their rules either.

Occasionally you see guns or armor from Fallout 3 in Fallout NV and it's not challenged because the actual assets (models, textures, etc.) are in the FNV game files but not actually placed in the game.However taking an asset from F3 that does not exist in the game files of FNV would break the rules as I understand them.

The only way TTW was able to share assets was by requiring a full licensed copy of Fallout 3 in the same directory as a full licensed copy of Fallout NV. That worked out very well, and kept Bethesda happy.