I know this question is not specific to WP but, it is kind of related. I run approximately 12 WP sites now (all in diff cpanel acct’s on a single server) and was reviewing possibly utilizing something like manageWP or infiniteWP to manage them all (eg updating WP core, themes and plugins).

Question is, is it possible to import existing WP installs into management software like this after the fact?

First you create an account on their website, feed them with your login-password and home url credentials for each blog, they’ll attempt to install their “wp worker” own plugin to the system, and from there on it’s managed from their homepage. If the wp worker installation fails, you’ll have to install it manually as a normal wordpress plugin, and try again on their website.