how to import one WordPress database into another, existing installation?

A site was established that brought together experts on a certain topic. Each expert was given their own, independent installation of WordPress, so they could have their own blog (I do not know why they weren’t all asked to write on one blog).

Having multiple installations is generating considerable expenses, especially as more blogs are added. When the site owner wants to upgrade WordPress, the programmer needs to upgrade it multiple times, for each of the various installations, and the various plugins must also be upgraded. For those plugins that need an API key, the upgrade is especially tedious, as each installation has its own key. There is also excess expense on the design side. Every time the owner asks the graphic designer to tweak the design, the design has to be tweaked in multiple installations.

To restore sanity to this situation, we are going to consolidate all the writers into one blog, using one installation of WordPress.

How do I import one WordPress database into another?