Yeah, you can't really do this in Marketo. Even SFDC has no out of the box capability to understand that Company=IBM means Account=International Business Machines or even just IBM.
What you could have done is something similar to what Anurag did by importing the list into SFDC by pulling a list of account IDs and matching them to the Lead IDs, but I think only Demand Tools would auto convert for you.
Another option is create a workflow that uses fuzzy logic to find existing related accounts. A client of mine does that and has a field that says "Related Account" on the Lead record. Then the rep can decide to convert it over.
So my recommendation at this point is to go into SFDC and create a lead view to find those leads you just uploaded and associate each one, one by one, to an account.