Or do you mean importing the registration fee information onto the Event record? Unfortunately IOM is unable to import to that field, so the fee information will need to be set up on the event prior to importing.
Please let me know if you have any other questions.
You can absolutely do this very easily, it's just that you would process the import file once and then make a change to the profile and import the exceptions file from the first import. The first method I thought of is a little different than Callie's, but conceptually the same. Here are the exact steps (and don't forget the last one):
1) In the profile, check the box on the "Constituents" node that says "Do not add new constituents if they do not match an existing record".
2) In the profile, change the "Participant is Constituent" field mapping to "Ignore"
3) Import file. Only those rows that match to an existing constituent will be imported, the rest will be in the exceptions file.
4) In the profile, undo changes from steps 1 & 2, and turn off Duplicate Searching (you can leave it on if you want, but no point)
5) Import the exceptions file, all remaining rows that were not matched to an existing constituent will not be non-constits.
6) Take the rest of the day off, you deserve it!
Let us know how it goes!