score:5
Google flights gives you a limit of 5 departure airports, while ITA Matrix https://matrix.itasoftware.com/ does not.
Steps:
Upvote:2
You could use Google Flights: https://www.google.com.br/flights.
It's not exactly what you're looking for, but you can select lots of citys to depart from to one or more destination.
As an example: Depart from: NYC, LA, SF. To: ICN.
You can mark flights and see the price variation on a daily basis.
In some cases, you can even book the flight through it.
Upvote:2
Kiwi.com's radius search is an excellent option. The maximum radius doesn't cover a whole continent but it's pretty big (e.g. can cover the US West Coast). Combine that with a string of multiple destinations:
If you are looking flights that include a connection, do note that some of the flights they sell are budget connections that have different risks than "normal" flights.
Upvote:4
Technically, skyscanner has such an option (disclaimer: I never used it and have no idea on how cheap/convenient it is). You simply type US in From field (and select 'United States' from suggestions list) and ICN in To, and choose 'Whole month - May' as departure/return date.
I suspect other widely used search aggregators do as well.