When you use the connector solution, you need to create a data source (the external database or file that you are pulling from). They you need to create an import\export rule. When view\preview this rule, During this stage, you line up (in the right hand column) what you are trying to match to in Altiris (in the left hand column).

This allows you to indeed compare what you are going to import, before you import anything. Also, it also allows you to indentify gaps in key data classes, which can be created prior to importing.