What is the scenario/use case (there may be a better way to accomplish this)? What IP's are you trying to track, and how are you discovering them? With NBTSTAT, you should only see hosts in your broadcast domain, or those that unicast with your host. Are you running this on the WINS, looking for cached NetBIOS names from clients who have made NBN registrations? The NBN cache has a default timeout of something like 10 minutes -- you might need to adjust the registry entry to cache them longer.
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\services\NetBT\Parameters\CacheTimeout
Your script that runs NBTSTAT should do the translation, if you want them done prior to indexing. I recommend having it provide both the IP and NetBIOS name to the indexer. This way, you won't have to rely on the DHCP logs, but can use them as another measure to ensure the validity of your data.
You are correct, Splunk can do the correlation between your DHCP logs and NBTSTAT script data, based on lease time and timestamp. You might even want to correlate them to WINS events, and SPlunk can do this, too.
As a side note, the Windows DHCP app might be of assistance here:
http://splunkbase.splunk.com/apps/All/4.x/app:Windows+DHCP
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