You can copy and paste nodes and transitions within a module and across multiple modules. You are not limited to one node and transition, but can copy a whole sequence of nodes. Hold SHIFT to select a range of Nodes. You can use your ordinary shortcuts like CTRL-C (CMD-C on a Mac) to copy and CTRL-V (CMD-V on a Mac) to paste nodes and transitions. If you are not familiar with keyboard shortcuts, you can also click on the context menu of a node, which shows you all the necessary commands.
Please note that the copied clipboard is lost when the module from which the clipboard was copied is closed. If you wish to copy nodes from Module A to be pasted into Module B, both A and B have to be opened in separate tabs in your browser.