Manirul,

If you have all the information you need in the source spreadsheet to create the Model, you can do this with two transform maps that you run in order, Model first and then the Asset. What this does is to ensure that the Model exists before the Asset is created. If the Model already exists, then the Asset will use the existing Model. Just be sure to properly identify the fields you want to use to "coalesce." These uniquely identify the Model or the Asset in the transform map so ServiceNow knows whether to merge or to create a new record.

Ben