Legendaries would be a little trickier – probably easier to just use the existing UI to create the variation that you want to rank.
For other items… theoretically it’s not that tough to take an item link from in-game and export it to the website in some way. The time consuming part is building the UI on both ends (addon, website) to allow it. It would be handy though. We’ll put it on the list of possible future enhancements.
In the meantime, if you aren’t able to create a variation of an item on the website, let us know and we can add it.