Thanks, I tried that but the only thing my part numbers had in common was their length, which they also shared with a lot of objects that didn't need to be added. I ended up using the following procedure:
save excel list as text file (space delimited) -> copy groups of space-seperated part numbers into "Keyword" field of Advanced Search with "Find: with any of these criteria" set -> select objects from search results -> paste into Affected Objects.
Unfortunately, this limited me to adding about 100 objects at a time, thus getting through one list required doing this twenty times. So I am still looking for a more efficient way if anyone knows of one.
Thanks again for the suggestion.