You can import a .csv file of your existing clients in the Client Explorer:
The file you use should contain header rows and you will map your columns in the spreadsheet to fields within SI. If you would like a template .csv file in which to paste your data, you can create one Client manually (if none exist) in SI and then click the Export button and choose CSV->Selected.
This will show you all of the available fields for a Client and you can choose to export as few or as many as you'll need for your data.
You can then delete the record from the spreadsheet and use this template for your import since all columns will automatically mapped.
You will need to map the columns in your CSV file to Client Fields in the SI. Some fields will be pre-mapped if they are an exact match or if they are a close match. When you click the Map link next to an item you will then be able to select the appropriate field or you can unmap any field by clicking the [None] button on the form. When done click OK.