#1 I’m not entirely sure what you mean by “field can be added into lookup on the fly” - if you mean you want to populate a lookup table using data from Graylog, then that isn’t possible.
#2 Also not sure - got an example?
#3 Yes, you can use a lookup table in pipeline rules (via the lookup function)
#1 there is no such plan to do so in the future. But feel free to open a feature request at github with example usage.
#2 I also would need an example to understand what you want to achieve.
#3 The lookup table is used to enrich your data. For example, you can tell via Tor lookup table, that a source IP is from a TOR exit node and you can mark that in your pipeline rule. And than you can decide in a alert condition if you want to act upon this marking. What do you need look up tables in alert conditions for?