If you have the latest update to Numbers, you can.
On the documents page, tap and hold a document until it starts to jiggle. Drag one document on top of another to create a folder with both. Continue to drag additional documents to the new folder as desired. To change the name of the folder, open it, then tap and hold until the documents jiggle. The name field will become editable. You can also do this when the folder is first created.