Redirect Old Single Install Directory to New Multisite URL

Dear All:

I'm about to migrate single installs of Wordpress in various directories into a multisite system in the root. Once I do that, I'd like to have the pages in the old directories redirect to their respective pages at the new URL.

In other words, if the old directory / link was this:

Then, I'd like the redirect to point here:

Is this best done with .htaccess or is there a plugin that exists that would solve this?

Many thanks for the help!