Making Cooper's kits more bitter

I'm still green to brewing and have what might be a simple question for the rest of you. I'm using the Cooper's kits right now due to a lack of time (3 kids keeping me pretty busy) but would like to tweek the flavour a little. I have stopped using Dextrose and gone to LME and dry hopping in a secondary. It has made a big difference in the quality of the beer. I'm only on my 3rd batch and I can tell the difference.

The problem is the kits still seem a little sweet to me. Can I boil some hops in the initial 2 litres of boiling water that I am using to mix the kit contents to add a little bitterness or do I need to just let the beer condition longer and the sweetness will fade. I have left the first batch a month now and it still seems sweet.

Can I boil some hops in the initial 2 litres of boiling water that I am using to mix the kit contents to add a little bitterness

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Yes. Boiling 10 grams of any high AA% hop for 30 minutes will add about 10 IBU to a 23L batch.

I've used two kits in the past to make a more bitter beer, works great so that's using 2 pre-hopped tins and no extra sugars.