Hello,
If you want to add a new external IMU, it is possible. You need to add a new backend in AP_inertialsensor and modify your hal to use it, and adapt the EKF gain for it… Quite a big work
But unless you are paying a quite expensive IMU (>1000$) you won’t have much improvement…
Default imu on pixhawk serie are quite good. At 1000$ it would be better to replace the whole controller …
Could you details what you mean by very big error ? I fear that you are thinking that the imu is faulty when it is the attitude that is the culprit. The attitude comes from fusing the imu, magnetometer, and gps in the ekf… But the task is complex as many parameters influence the accuracy : raw parameters, frame response, motor response, electromagnetic perturbations etc…
Like I said, it is more likely that only use better imu won’t improve much the system. Investing in a better GPS could be a better and easier start even if it won’t solve everything.
Many companies use only a pixhawk like to attain sumetric accuracy (and dji got the same sensors too) so you need to identify the correct source of issue on your build before investing big money on theorical improvement.
I invite you to create a post on copter section with the maximum information on your build and a dataflash log (the one from sd card). That way many expert on the forum will be able to help you to improve your build !