Currently, the Batch feature only supports exporting the designs in a batch. They do not save to your workspace. We may look to allow saving batch designs in your workspace in a future release.
If needed, you can export a batch and then import an image from that batch back into your Photroom workspace. This works in the same way as importing any photo, so you are importing an image rather than a design with selectable elements. To learn about importing a photo, see Create your first Photoroom design.