Although not officially supported by us, the Outlook CSV import has a few hidden options that allows you to import your own CSV file. All you need to do is edit it in your favourite spreadsheet program, and upload it using the Outlook CSV importer.

The fields are as follow:

• Company
• Account Number*
• Web Page
• Description*
• Business Phone
• Business Fax
• Business Email*
• Business Street
• Business Street 2
• Business Street 3
• Business City
• Business State
• Business Postal Code
• Business Country
• Title
• First Name
• Middle Name
• Last Name
• Job Title
• Birthday
• Person Description*
• Home Phone
• Mobile Phone
• E-mail Address

The ones with *s at the end aren't ones used by Outlook, but we've included support for them anyway.

I'd reccomend that if you have a lot of data, you might want to test importing a small subset of it first, just to check that the column-mappings are as you expect. Mass-deleting isn't currently supported, though we do plan on introducing it at some point.