When you go to map columns for an item import, one of the choices under “Item-specific data” is “Item set (by specified property)”: that is for choosing a set for the item being imported. You have a dropdown choice there of what data you have in your sheet to select the set: “Internal ID” is probably the simplest if you know the ID numbers of your item sets, but you can also use the Title or Identifier properties, or any other suitable property.