The only way which I see is to first download them all into a special directory used only for this purpose; then perform a "dir" (resp. "ls -l", depending on the operating system of the PowerCenter server machine) on all these files and use a PowerCenter mapping (which reads all those file names as its source) to find out which file names fit the date requirements; save only these dates to a target flat file.
Then in the next step use this flat file as an "indirect" file source (i.e. a source file list) for your actual load mapping.
I know, this is rather short, so please don't hesitate to ask for any additional detail you may need.