There are 3 ways an existing site (using whatever tech) can be involved -- 'integrated' depends on the depth of integration:

• Using first-class RESTful web services -- this is the new, sexy thing that will come out of this project, but it comes at a development burden for the implementers of services. However, since we're building service resources for TN.org (D6) and IIRS (D7), this work can be adapted for these other Drupal sites relatively easily. For WordPress and other platforms there will need to be some care in ensuring the data and services work -- but they should do with a little effort.
• Feeds -- They can get their data in and out via 'classic' RSS feeds. The importers/views will need creating where they don't already exist and might be bespoke per pair of site needing connection. This is the least preferred options as feeds are 'dumb' compared to web services.
• Widgets -- They can show initiatives in their country on their with our widget. They can also add them via the widget to bring them into the international view, and/or use some volunteer time keeping things in sync or migrate their existing set of data... Given were the Widget module is going, this widget-based service could extend to many other variants, views and tools relatively easily. This allows other sites to effectively outsource part of their database and site features to TN, using widgets as drop-in replacements for bits of their pages. Maybe!

Clearly the automatic, intelligent web service approach is best by a long way. We're in a kind of 'if you build it they will come' situation, and just because existing sites have always been islands doesn't mean we can't help them join up. Carl Shuller at TUS was always keen on combining databases/efforts, and when working with me has seen the huge improvements D7 brings over D6 with the Daily Acts site...

Again, if we build Drupal-focused web service coverage that's a big chunk of the backbone of the network built, and sites covered... Perhaps then we can start negotiations on migrating, extending or simply connecting the others via these standard services and data.

Hopefully the D7 National Hub offering will gain support and may even one day lead to these incumbents migrating to using our tech -- we just need to make it compelling, easy and cheaper than developing their own.