If you are using blueprints, check out the SaveGame system which will allow you to write data to a file:
https://docs.unrealengine.com/latest/INT/Gameplay/SaveGame/Blueprints/index.html
If you are using C++, you could try something like this:
A new, community-hosted Unreal Engine Wiki - Announcements - Unreal Engine Forums,Read%26_Write_Any_Data_to_Compressed_Binary_Files
I either case you are just going to loop through the actors and write their transformations to a file. I am assuming you want to use them later to reload a procedural level.