You are right, it is indeed currently not possible to use one of yEd's swimlane or pool node templates for groups when importing data from Excel.
However, it is possible to use the BPMN "Group" template. Excel import uses the default group style template when creating group nodes . Thus you can switch your default template from one of the templates in the "Group Nodes" palette section to the "Group" template from the "BPMN" palette section. Changing the default template may be done by double-clicking the desired template in the palette section. If the change is successful, the background of the palette entry will turn a dark blue.
If you need different group styles based on data in your Excel sheet, use "Tools" -> "Select Elements" to select all nodes with a specific data property value, then right-click the desired group style template in the palette to open the template's context menu and choose "Apply Type". Unfortunately, it is currently not possible to use the properties mapper for this task and thus you need to do the "select, apply" routine for each group style you want to use.