Exporting Customer Custom Fields
Step: 1: From the Administration Index, select Customers > Custom Fields
Step 2: Click the Export button on the bottom of the page
Importing Customer Custom Fields

Ensure that your document is saved as .csv
Step 1: From the Administration Index, select Customers > Custom Fields
Step 2: Click the Import button on the bottom of the page
3. Click the [Choose File] button in the Data Import dialog box
4. Find your .csv file and click Open
5. Click Close on the Import Complete dialog box