I think very small in this case (assuming you have a seperate import table that you do not use for anything else) is an overstatement.Just so you are aware, you should not have any trouble doing this with an autonumber. Sorry I have scared you, but the odds of it NOT working as planned are actually very small. But you do need to be aware of the risks.
I agree there is some form of risk using an autonumber, but 99.9% of all cases it is sequential and 100% full proof in this type of situation.
There is an alternate problem which the OP may not be aware of... The nature of a query in any database is RANDOM by default and definition.
That is UNLESS there is a specific "order by" statement in the query beeing given to the OP, the order of the query is RANDOM.
Again in 99+% of all cases it will be the order of data-entry that is returned, but there is NO GUARANTEE, unless you have an explicit order by statement.
Even if you have then an order by on the date/time of the record, there is no guarantee you will get the right order for date/time that are equal to eachother.
NOW!!! NOTE !!! This is basicaly exception of the exception that by exception will cause an exception, but NO GUARANTEE can be given unless you can GUARANTEE IT.
If the order in the file is 100% guarantee... then there must be some PK there with an ORDER BY... else people are BS-ing you