You can add the syrup post ferment...you have to backsweeten the SkeeterPee anyway, so measure the syrup and calculate it into the sugar you will backsweeten with. FWIW, you can add the strained fruit when the SG has dropped by 2/3 if you wanted--reserve syrup for after wine is dry. This will provide more cherry while still fermenting the fruit. Heck, we seem to add fruit at all stages when making this. There does not seem to be a right or wrong. Boy oh boy, cherry lime!
You are using lime juice concentrate, not lime juice and following Skeeter Pee recipe?