If you have the starting private key, then you can calculate the final private key. It's simple addition.
Yes. Add the privatekeypart to the starting private key. The result will be the final private key.
Think about it like this...
If I tell you that the privatekeypart is 75
AND YOU ALREADY KNOW that the starting private key was 153
then you can calculate that the final private key is 228.
If you don't know what the starting private key was, then you won't know what to add 75 to.