Unfortunately the ability to import old posts from websites or question dumps is neither available nor advisable.
On a Stack Exchange site, the content is more than just a collection of text. The knowledge curated on an SE site has both ownership and a healthy dose of vetting by the community charged with keeping it up to date. When you start with a dump of legacy content collected elsewhere, you take away the ability for the new community to ask and answer those questions themselves. The site has a lot of posts, but without imparting any of that reputation needed to take care of that content. It's mostly abandoned.
A site installed with pre-loaded "answers" starts with a foundation of little to no ownership — the original authors are long gone, usernames are greyed-out as 'anonymous', voting is absent, internal links are broken, user notifications requesting updates or improvements go nowhere… and the foundation of the site will is a collection stuff which doesn't quite seem to "fit" the rest of the site.