It is possible from search request?
Use following template for querying
"some search" | inputlookup append=t test_lookup_table | "what you want to add to the lookup table" | outputlookup test_lookup_table
This solution good work for usual lookup. For redis lookup it is not working, unfortunately...
You could build a custom search command to do this.
If you do, please put in up on Splunkbase.
I will try to create such a search, but I'm new to programming and Splunk, unfortunately.