Do you want the hash of the file or the hash of the filename? The filename you can hash easily enough like | eval hash=md5(filename_field) . There shouldn't be a need to index that - you can run it at search time.
If you want to calculate the hash of the file itself you'll need to do that with a scripted input - the universal forwarder by itself won't pull that from the event log (unless there is a windows event that contains that information)
There are most likely many answers on here about the best way to do malware monitoring - also check out the channel #security on the Splunk slack for more advice
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