First, I'd suggest trying the insEKF without handling the lever arm and see if the performance is adequate.
If that doesn't work, you can build a custom sensor plugin for the insEKF to handle sensors at a lever arm. There are several examples of custom sensor plugins for the insEKF. Here's a good one to start with : https://www.mathworks.com/help/fusion/ug/design-fusion-filter-for-custom-sensors.html
