is there any possibility to get the raw IMU data in the SDK? It seems, that sensors (pressure, gyro, accelerometer,...) are connected only to the LLP and its raw measurements cannot be reached from the HLP.
It is not possible to get the real raw measurements of these sensors. Nevertheless, you can access angvel_..., acc_..., and H... in the RO_ALL_DATA structure in the SDK. This data is only scaled slightly and offsets/biases have been removed. Apart from that, these fields contain the data directly from gyros, accelerometers, and magnetometer.
If you want to write your own sensor fusion, this data us usually much more suitable than the raw data.