Heat the milk gently in a large, heavy saucepan until it starts to steam. Remove the pan from the heat and let the milk cool to room temperature.
In a small bowl, whisk together the starter or yogurt and about a 1/4 cup of the lukewarm milk until smooth. Whisk this into the saucepan with the rest of the milk.
Transfer the milk to a measuring cup or bowl with a spout and pour carefully into seven 7-ounce yogurt jars (make sure these are clean and dry). Arrange the jars, without their lids, in the base of the yogurt maker and cover the base with the clear plastic lid. Plug in the yogurt maker and set the timer for 7 to 8 hours, depending on how firm you like your yogurt.
When the yogurt is set, screw the lids onto the jars and refrigerate them for at least 3 hours.
Once the yogurt is chilled, stir a few spoonfuls of the fruit puree and a bit of maple syrup (taste as you go) into each pot of yogurt.
Combine the fruit and sugar in a small heavy saucepan and add 1/4 cup water. Bring to a boil over medium heat, cover and lower the heat until just simmering. Cook for about 5 minutes, until the fruit starts to soften and release its juices. Mash the fruit and strain it through a fine mesh sieve. Set aside to cool and then cover and refrigerate.