The solid foam filled tires on a skidder would be a very bad thing. The ability to distort and conform would be very limited, resulting in much less traction. The ride is also much worse with foam filled tires. If you feel the need for more tire weight/lower center of gravity, go with a liquid fill.
A plastic nozzle off a quart gear oil bottle will thread onto a garden hose and can be trimmed to fit over the valve stem tight enough to get a lot of water into a tire. Just rotate the valve stem striaght up and take the core out. Make sure you add enough anti-freeze.
With water filled tires, be carful with you drive train, it will be under a lot more stess.