I know little about Yara (in fact, nothing until a few minutes ago); it seems it is some sort of a malware identification mechanism operating on files and filesystems (at least primarily).
If that's the case, I'm not sure Splunk can do much with Yara rules directly. First, I doubt you are indexing malware samples or the files malware might be in against which you would run the Yara rules. Second, If you actually were ingesting malware directly into Splunk, or something like that, then I'm not sure how you'd get Splunk to read the Yara rules and apply them.
But all is not lost!
It seems Yara has an executable. I can't quite determine what output it creates, but that hopefully means it just dumps output to standard out. In that case, you could redirect it to a file and read that file into Splunk and possibly do wonderful things with it. This should certainly be easy enough to test.
It also can be run from python, which means it might be possible to run it as a scripted intput. Quite a bit more work would probably be involved in this, though the payoff may be great if you can figure it all out well enough.
