When you are setting up the promoted properties in the InfoPath designer (Tools | Form Options, I believe) there are choices for what data to get promoted when you have repeating data. One of the choices is merge, another is first, etc.
Keep in mind that you cannot actually promote repeating rows of data, you'll need to do some coding to populate a SP list if you want something like this. Instead, merge will take each repeating row and make a concatenated string from the field. For example, if you have 3 rows like this:
Red
Green
Blue
then your merged SP column will be "Red, Green, Blue" (actually, I'm not sure whether the commas will be in the string or not--but you get the idea).