found the following on the web under this link Wifi Data
First, to backup your Wii Fit save data, insert an SD Card and copy
the Wii Fit game file to the card. You also need to Transfer all of
your personal MIis to a Wii Remote. If you don’t, the characters will
turn into guests on your friend’s console.
On the Mom’s console, go to the Mii Plaza first, and Transfer all of your Miis to their Wii (from your Wiimote).
Then, insert a blank SD Card. Copy their Wii Fit save data to it (if
it exists). Then, erase their data from their Wii (make sure it’s
backed up on their SD Card first!).
Remove their SD Card and insert yours. Copy your Wii Fit data to
their Wii. Then go ahead and start and play Wii Fit with your save data.
You can simply reverse the process to transfer back.