1. I would NOT advise to do this in Prod directly (unless the SPO was created in Prod ofc), but to follow the normal development path. If you don't, you will never be able to transport the SPO from Dev again
2. Just go into 'change' mode of the SPO, choose option "2. Change Partitions". On the right hand from the screen you can then add the desired partitions. When done, just click "3. Start Activation"