FO5 tyres are pneumatic - but that's the last thing to consider.
Firstly is the slab lifting boom design such that the slab is a swinging load? If so then for safety reasons the capacity must be reduced by 20% to 10800 lb or the equivalent load at the laod distance,
Secondly, at the point where the steer tyres lift off these forklifts will be overloaded by around 50% where the load is not a swinging load (or around 80% if the load is swinging).
Thirdly only the manufacturer knows if the forklift is limited by forward or side tipover. If its limited by side tipover then the capacity cannot be increased by adding weight to the steer tyres.
Fourthly any addition of weight at the counterbalance reduces side stability because the centre of mass moves backwards and is then closer to the edge of the stability triangle - so even if the forklift in its supplied form is limited by forward tipover adding weight to the counterbalance may result in side stability being the limit.
Finally forklifts are a dangerous item of plant which are associated with too many deaths and serious injuries - any suggestion that they can safely be "modified" without professional advice from the manaucturer dispays a lack of appreciation of the respect with which forklift operations must be given.
As for tyre ballast if approved - for 825-15-14pr tyres, they can only be 80% filled, and with calcium chloride at a specific gravity of 1.3, a wheelbase of around 2250 mm, and a load distance of around 620 mm, the maximum increase in capacity would be around 5%.