Well, to see if I can clarify your position:
You have multiple subdomains each using a different CMS/system to run and you want to bring them all under one CMS/system during a site restructure.
You could, most likely, run each of the systems in their own folder (e.g. /blog/ is powered by Wordpress, /shop/ is powered by Joomla) but I think this is a case of better to do something well if you're going to do it at all and bring it under one main CMS.
I recently had to do this from a bespoke CMS to an open source one and, as there was no real way for them to talk to one another, it was unfortunately a copy and paste job. I only had about 2000 pages though, so would seem less likely you can manually do it with 100,000
If you can export your content in say XML or a database and then parse it into a format that your main CMS would recognise that may be one way of doing it.
Now looking at your response below you've no coder/programmers as such working for you, so what I've suggested may be a bit beyond what you can achieve without hiring one.
Just to be clear though, you cannot have different parts of a site hosted on different servers, so parentcharity.com/sales and parentcharity.com/community MUST be hosted on the same server.
The language that powers each of these sections CAN be different, but you'll most likely have to speak to your current providers and ask if they can move it to the same server. If it's a bespoke solution they may not be able or willing to move it to a server they don't control.
Let me know what I'm missing