Yes, you can give about half cranberry juice/half water provided your guinea pig likes it - by far not all piggies do! there is still a fair amount of sugar even in no added sugar cranberry juice. When syringing water, rather have a session once or twice daily where you offer your guinea pig as much water as it will accept in order to encourage it to make one larger pee. Please never force a guinea pig to drink when it doesn't wat to and be very careful never to syringe more water than it can hold in its mouth (ca. 0.3 - 0.5 ml, i.e. 1/3 - 1/2 of a small syringe) to prevent it from going into the lungs and nose. Wait until it has swallowed the previous lot before you give more. You may not get as far as 10 ml in one session with your piggy; a fair number won't take as much.
If bladder sludge has built up, syringing extra water will not cure the problem, nor will cranberry juice. In my own experience, cranberry juice has never made any difference in the several guinea pigs I have tried them with. We have dropped it from our recommendations several years ago as one of things that have made the rounds but have not stood the test of time.
What can help to protect the bladder is glucosamine (ideally vegetarian or vegan glucosamine) or a glucosamine based cat bladder food supplement. The bladder wall has a natural glucosamine coat. It is usually getting rather irritated inflamed from the constant sloshing around from stone or sludge, especially the painful pressure when peeing. The supplement (it is classed as a food supplement and not as a medication and is prescription-free) does help with supporting the bladder. It is being sold in the UK as cystease.