Option "Label Text" in section "Nodes" on tab "Presentation" applies to both, normal nodes and group nodes. For group nodes, the label text applied in this way will be used for both, opened and closed state of the group.
If you need a different label for closed states, add an extra column for the corresponding texts, import your data, close the groups in question and run properties mapper to set the appropriate label texts. Note, properties mapper does not recurse into closed groups. If you have groups in groups in groups etc you will have to close all leaf groups first and run properties mapper, then close the next level of group nodes and run properties mapper, and so on until all groups have been closed and their label texts adjusted.