Move an Existing Blog into WordPress Multi-Site

Move an Existing Blog into WordPress Multi-Site: "to copy all of our old blog data (e.g. posts, pages, tags, categories) over to our new site. I tested out two methods -

1. Import File Method

I imported the export file I created earlier using Tools >> Export. This method gets me all of my posts, pages, and custom post types. It also gets me all of my tags, categories, custom taxonomy terms, and comments.

However, this method has the following weaknesses –

Thumbnail and featured images are not transferred.
iframes in posts and pages are lost.
Post and term metadata are lost.
Links to media files in posts and pages point to old links (i.e. they point to wp-content/uploads and not blogs.dir/5/files."