Unfortunately, it is not possible to extend a pulse that has already closed.
However, it is possible to extend ongoing pulses one extra week if they were only one week long at the beginning, (the setting has to be unlocked first, send us an email to help@andfrankly.com). Pulses that are already two weeks long cannot be extended any further.
Please note that only pulses that had you have created yourselves can be open for two weeks, read this guide for instructions.