Hi sangbum kim,
In the case you mentioned, i.e., Excel Import, the cloumns must have to be mapped with the attributes of entities.
Within Import mapping, when you are mapping the columns, mendix will automatically creates the entities with the option Map Automatically.
To have to create entities by excel try from start i.e., at Creating new app select Start from Spreadsheet, so that all done automatically.
If you want to create entities from an external source, you would need to use the SDK. Here's a nice blog about how you would be able to do that.