The only website I'm aware of that does this is kiwi.com. All the others allow you to search from X to anywhere, but not from anywhere to X. The best Skyscanner can do is to search from Country to X (e.g. Spain to Paris) but it doesn't search at the regional or continental level. Kiwi allows for continent (e.g. Europe to Paris) and regional (e.g. Eastern Europe to Paris, Southeast Asia to Paris) searches. You can always then double check prices on other websites once you have a flight in mind. Hope that helps.

I spent ages researching on this recently when I was trying to book a flight back to Australia from Europe, but I didn't really mind where in Europe I left from. Kiwi was the only one I found that allowed for it.