Sorry, friend. I understand where you're coming from, as my worlds were largely created outside WA and I have been using it (for years now, sporadically) to centralize, finalize, and expand the details of my worlds and stories. However, copy-paste is a thing, and unless you want all generic articles with no formatting or miraculously already have your information in a layout that would mimic the article template you're using, you're still going to ultimately have to copy-paste or otherwise largely manipulate your data to rearrange it nicely anyway.
What data? What format? Spreadsheets? Text files? Campaign Cartographer maps? Realmworks files? Homebrewery markup? They all have different formats & need development time to create import tools. I suggest resubmitting this request with an indication of the sort of the formats you're thinking of, so people can evaluate properly whether this will be useful to them, and vote fairly.
Without specifying what data (and in what form) you want to import, this is far too vague of a suggestion.
There are thousands of ways that your data can be stored. Each needs to be implemented separately. So if you want this to happen for your use case either make a specific feature request or find someone who can implement it for you. Or learn to do so yourself.
I feel your request is too vague. What are you asking to import data from? I agree that I import a lot of data into World Anvil from my personal notes apps, but what are you wanting to accomplish that can't be done by copy pasting?
You can already do this via the API.
I think this would create too many external issues not to mention the complexity of what you ask.