Hello heymoe -
as long as you have the correct category selected for your items, you can include the custom item specifics in your csv file using the exact name of the specific.
To do so you would create additional columns for each specific you wish to include and use the column headings in the format like this:
Where "Brand" is the item specific you wish to include.
Should you encounter any difficulty, please feel free to file a support case letting us know where you ran into trouble, so we can have a look into it for you.