Since it's already in a keg and connected to the gas I wouldn't add yeast or fermentables now.
One reason for lagering is to clear out the sediment and this would stir things up again.
If you still want to bottle, check out this thread:
Carb the beer up to the way you like it.
Then turn the gas down to 5 psi and fill cold bottles with the BMBF.
I've carbed wheat beers up to 3.5 volumes (20 psi at 33F) and bottled with this method.
I'm not sure, but isn't a liter of candi syrup like 3 or 4 pounds?
If so, that's way too much for a 5 gallon batch.
Depending on the amount of residual carbonation from fermentation,
priming calls for 4 to 8 ounces (by weight) of corn sugar.
Candi syrup has some water in it and is less fermentable than dextrose, so it would take more.
There's probably a calculator on the interwebz that will give you a precise amount.
And you need to be precise - too little priming means flat beer, too much means bottle bombs or gushers.