Assigning a 16 bit number to two bytes of a buffer
Using a union was sufficient for my purposes.

  out_buffer[7] = 0xD0;
  out_buffer[6] = a_x_filtered.byte_data[1];
  out_buffer[5] = a_x_filtered.byte_data[0];
  out_buffer[4] = imu_union_ACC_X.byte_data[1];
  out_buffer[3] = imu_union_ACC_X.byte_data[0];
  out_buffer[2] = imu_union_ACC_Z.byte_data[1];
  out_buffer[1] = imu_union_ACC_Z.byte_data[0];
  out_buffer[0] = 0xC0;


Here's the union structure:

typedef union
{
  uint8_t byte_data[2];
  int16_t g_x;
}imu_union_t;


And this is how i declared it:

imu_union_t a_x_filtered;
imu_union_t a_y_filtered;
imu_union_t a_z_filtered;