There isn't an easy way but it is possible. You will need to copy all your sprites and sound files in to your main game from the other games and then open each game into one main game and copy the event sheet and layout to a new named event sheet and layout and then erase the original event sheet and layout.
If you have sprites or sound files with the same names in games but are different you will need to change the names before transferring them.
It is something C2 needs to add to import an event sheet and layout but it is not possible any other way at present that I am aware of.