Is not possible to add a protection leg to an existing MSP 1:N.
There is not any "modify button".
As is not possible to modify, if you need to add a protection leg you have to delete the MSP 1:1 and then to recreate again adding the new leg to the protection.
MSP 1:N has been create in this way. You only can create and delete it.
If a MSP 1:N exist and you want to add other board, you have to delete the MSP and then create it again (including new board) instead of be able to add the board directly (customer has to spend more time deleting and creating again than if he would able to add directly but this function works in this way).