The only requirement for creating an appeal is that it have a name. In the context of importing constituents to an appeal, you need to have a value in the "Gen. Appeal Name" column/field.
NOTE: When loading appeal gift records, you can associate gifts with appeals by mapping the "Gift appeal name" column to the "Gift Appeal Name" field, as shown in the field mapping screenshot below.
Custom categories and attributes (different types of fields) can be added to an appeal in LGL; during import those custom fields will be available if you choose to display them and you choose the appeal.
Now you will be able to match the columns in your import spreadsheet to the appropriate custom category/attributes in your import mapping.
Be sure you also assign a rule by using the "set defaults" rule for "Gen. appeal name", mapping it to the appeal you're importing into.
NOTE: It is possible to import to custom fields for only the one appeal you select in the Appeal and event connection section of your import mapping page.