Hmmmmmm
I wouldn't / haven't done it, but what you might be able to do is set the report source to a sql clr table valued function, which takes as a parameter a full path and file name, then parses out the data?
Then, according to your comments, what I would do is take the csv of ints and pass it to a sproc. The sproc would then turn the csv into a temporary table using a table valued function. The table produced by the table valued function would then be used in your query. I would avoid this entire scenario if I could. If you can provide the user with list of valid parameter choices, so much the better. Or if that csv is a regular, slowly changing list, perhaps it is better to make it a separate report. SSRS allows you to 'copy' or 'link' ( i forget the term ) to base/master reports. All you're really doing is changing the set of input parameters. The basics of the report remain the same.
See:
Good LUCK!