If I understand the question, this may help.
| inputlookup lookuptable.csv
| fields a
| mvexpand a
| map search="search index=$a$"
If the field is not multivalue then mvexpand will not do anythihg to it.
If I understand the question, this may help.
| inputlookup lookuptable.csv
| fields a
| mvexpand a
| map search="search index=$a$"
@richgalloway - thanks Rich. May I ask, what if some of the values are multivalued, pipe-deliminated values. How may I make $a$ single-valued?