If it is all bottled then it may well be worth leaving the bottles for some months and see how the secondary fermentation progresses. There may have been an incomplete primary fermentation that may add to the carbonation.
However if this is the only brew in town and every bottle opened is flat then it might be worth reopening and adding sugar. That is probably best done in solution and measured into the bottles with a syringe. Make up the weighed sugar with a known volume of boiled water (do the math!). Have the bottle caper and a supply of caps handy!
IMHO an easier method to prime beer for bottles is to mix it all in one go into the beer. Rather than spooning an approximate volume into each bottle, dissolve all the sugar in a minimal amount boiling water and then dilute with some cold water and add to the beer to be bottled. If the beer is over the trub then it is best to pour (or sip[hon if cautious) the beer off the trub and onto the priming solution. A quick mix will mean the beer has been uniformly dosed with dextrose and there is no variation from bottle to bottle.
As a guide I use 150g of dextrose in 24 lt (approx 5.5 gal.) of beer.