While I think going the ldapsearch route will make more sense, I'll answer your other question as well.
You can certainly configure the Windows forwarder to pick up a CSV file. Doing so would create events in Splunk--not a lookup table. To do this, you would follow the steps in the indexed field extraction section of the Getting Data In book which deals with "files with headers". Since the forwarder doesn't have a UI, you'll need to edit the inputs.conf and props.conf files. If you are new to this, The easiest route would be to quickly install Splunk on your workstation (you can set the services to manual so they don't run all the time if you want). Then in splunkweb, go to add a new file input, and it'll walk you through the steps of creating the right props and input settings. You can then put those files on the forwarder with very little modification needed (such as the path to the CSV file).
Once you have that part figured out, just have the forwarder watch the file, and it'll update the file as soon as your script writes it out. Seconds later, you can search that data in Splunk. Now, these won't be lookup tables. That may or may not matter for your scenario. Next step for you would be learning the right search commands to compare these fields and so on.
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