Sorry, Linux isn't really my forte, but usually (ie in other *nixes) you can edit an existing crontab directly using crontab -e (though it's not really recommended, as it's better to copy the crontab with crontab -l > newcron, edit newcron to ensure no errors and then activate it with crontab newcron, but that's getting into deeper water than perhaps you like).
You can pick up specific file extensions in the find by adding -name '*.ext' to the above, ie:
find /path/to/dest -mtime +30 -name '*.ext' -exec cp {} /path/to/your/folder \;
replacing ext with whatever your extension is.
You do need to make the script executable. Use:
chmod 755 scriptname
to do this.
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