I've had a look for a solution to this and surprised it's not easy to find.
I'm converting and old site into WordPress on behalf of a charity, and one of the things they wanted to ensure was that pages from their old site go to the new post/pages in WP. Therefore I've created a custom field in WP to capture legacy URLs (eg the directory/filename of what this post was on the old site).
I assumed there would be a plugin that I could add that would handle these, so that if visitor goes to the old URL they are redirected to the new WP post. However I've yet to find anything (all the redirect plugins seem to be the other way around - push people out of WP).
Any ideas of a simple way to implement this?