When constructing a RateCalculation instance (such as IborRateCalculation) you can provide a gearing/multiplier to the builder. See IborRateCalculationTest for an example.
The example has a multiplier schedule but you can use a constant value by calling ValueSchedule.of(double value).
However, it may be possible to tweak Ibor-to-Ibor swaps which are realised in the convention IborIborSwapConvention. One leg can be used for the swap index leg by creating a customised IborIndex, whereas the other leg is used for the Ibor leg with gearing as Brian mentioned.