Are you referring to importing data from spreadsheets? If so, subgroup work the same way as groups. E.g. suppose you have node IDs n1, n2, and n3. If you specify n2 as group node ID for n1 and n3 as group node ID for n2, you will get a top-level group node n3 with a group child node n2 and a normal node n1 that is the child of group node n2.