Ok, you have two tasks here: convert the output form your load cell into a linear voltage proportional to the load on the load cell, and then convert that voltage into a current of 4-20 mA.A current generator is fairly easy to make - an opamp driving a transistor that gets its feedback from the emitter resistor of the transistor. You can configure the opamp as a summing amplifier with a preset to give your base 4mA and the processed load cell voltage added.Texas Instruments do make some current generator ICs especially for this - XTR117 - but you may have to design your own.It's easier if you have a separate power supply but it is possible to generate the sensor and current generator power from the loop power. I googled "4-20mA current generator circuit" and got plenty of circuits, but I think I'd just use these for inspiration...
