I'd like to know if there is a way, too. This was one area where WP seemed a giant step backwards from iOS. Not only do I have to choose between no punctuation or the word "period" showing up every sentence, Cortana likes to cut me off in mid-sentence if I pause the least bit when using the voice assistant.
Say a sentence. Wait for Cortana to ask you to add more. Speak your next sentence. Wait to add more, etc. It could be a long process. Alternatively you could say "and" instead of waiting between "adding more".