That's actually what I did also, I created a full backup of the wii, transfered everything over to the wii u then restored the wii with the backup.
I want to say that you can chose, but I don't think it was really an option...I know that it warned me that some things couldn't be transfered but I don't remember being able to pick and choose what did.
After the transfer it completely wiped the wii clean :/