With ImportOmatic, you can easily import Organizations and Individuals in the same file. As Melissa indicates, IOM determines if a record is an Individual or an Organization based on data being present in one of two fields:
Constituent Field ---> Last Name = Individual record
Constituent Field ---> Org Name = Organization record
An easy way to add something to only new records is to leverage Default Sets from Raiser's Edge in your IOM profile. When used in IOM, Default Sets are only applied to new records created during the import.
Default Sets are configured in RE (Tools on the menu bar, Organize/Create Defaults), and then you select them for use in your IOM profile(Defaults ---> Default Field Values ---> Choose the desired Default Set(s) here).
If you need to add Attributes to both Organizations and Individuals, then you will want to create one Default Set for Individuals and one for Organizations, each containing the desired Attributes. You will then select a Default Set for each record type in the Defaults section of your IOM profile. If you already use Default Sets with your import, you can simply add the desired Attributes to the ones you are using now.
Please let me know if I can be of further assistance.