This suggestion is already noted and we have plans to incorporate this into a future version of the game. I too want to send multiple offers to multiple players at the same time.
I grow weary of having to anally make diplomatic offers day after day after day. I'd like to be able to make a diplomatic offer and have it occur each day for a set number of days. The current system allows for one offer to stretch over multiple days but it does not have the same effect as making that same offer the same number of days as it seems that each day a new diplomatic offer must be given in order to mollify the AI's. The same offer that say gives cash for multiple days does not mollify the AI except for the first day. I would like to see diplomatic offers be able to be set for multiple days so that each day that same diplomatic offer goes out as if it were a new one each day the same way it would be if we did the same offer day after day for multiple days.