I think you can just set the I2C pins in MyBoardNRF5.h without remapping the pins in MyBoardNRF5.cpp.
Look in MyBoardNRF5.h, under the Wire Interfaces section. Set SDL and SCA to 30 and 31 as appropriate for your board.
@zeblong, you can generate these from the board files, however i'm leaving this one for @Ivan-Z to discuss with you as this is his device and he knows all details regarding this.