You should be able to import a phonebook in CSV format into providers such as CallCentric or Voip.MS or various others.
What specific issue have you run into?
How did the issue of doing modifications come up?
Just make backup copies of any CSV files that you want to work with, and plunge into it.
No harm will occur, because you will have your backup copy. So at worst you won't be any worse off.
This may help too.
»www.ehow.com/how_6901359_edit-cs ··· ats.html