If you import them as a lead, they'll be on the system as such and not really in the right format to do what you need.
My preference would be to write a quick on-demand workflow against the Contact record to create a Note where the title is something like "Saw Contact at exhibition", the description can be whatever you want but make sure the Regarding is set to
the Contact record. Then just run this workflow against all Contacts that you saw. It will then create a Note against each of them.