According to the FTC, credit freezes do not block existing creditors from accessing your report (or adding/updating records to it). Nor do they impede collection agencies from doing so, if they are acting on behalf of an existing creditor.
http://www.consumer.ftc.gov/articles/0497-credit-freeze-faqs
I found this by googling CREDIT FREEZE COLLECTION AGENCY -- the page gives lots of other good information about helping a consumer understand what a freeze is and what it does. Google knows everything.
As far as what a freeze does to negative information falling off at the appropriate time, it does not impede this, since the CRA is the entity that removes such negative data, not the creditor.
It is possible for collection agencies or creditors to add something to one's report after the date at which it is supposed to fall off, but (a) they are not supposed to and (b) the CRA ideally should have internal software that prevents this. But it's still possible I suppose (though uncommon). In that event you have every legal right to demand its prompt removal.
Finally, note that the "7-year" window governing negative data on credit reports is a different animal entirely from whether the collection agency can win a lawsuit against you for its collection (statute of limitations).
And even if the SOL has passed it doesn't mean that the collection agency couldn't still attempt to collect it -- even file a lawsuit for its collection (though that is rare). It just means that you would automatically win if you appeared in court and pointed out to the judge that the SOL had passed. I have a few huge CC debts from the 90s that once in a great while still rear their heads in the form of a CA contacting me. In which case I have to tell the new CA that the SOL has passed and that I am asking they never contact me again.
