I assume you are opening an IGES in Creo which will assign the name from data in the imported file by default. You should be able to change this name in the import dialogue box for the top level file at least. If you want to control the names of all the parts in an assembly you will need to rename them in Creo after importing the assembly file.
I would suggest that you use your start part templates to import data and not open them directly if it is something that will be used to create design data. Using this method will enable naming the file before importing any data.
You should see this in part mode and something similar in assembly mode as well when opening an IGES file as a new model.
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