A copy category. Here's one I use, specifically to copy categories to tags:
Code:
replace_metadata:
## tagcategory standardization
tagscategory=>(Portal|Half-Life)=>Half-Life & Portal
tagscategory=>(Mass Effect: Andromeda|Mass Effect Trilogy)=>Mass Effect
## Sorts anything other than a few selected categories into "Fanfiction.Other"
tagscategory=>^(?!.*(Half-Life|Portal|Mass Effect|The Elder Scrolls|Crossover)).*$=>Other
## Adds Fanfiction. to the beginning of tagcategory
tagscategory=>^(.*)$=>Fanfiction.\1
# 'add_to_' concats on to existing param, thus the leading comma
add_to_extra_valid_entries:,tagscategory
# tagscategory is a copy category for Fanfiction.category in tags
include_in_tagscategory:category
include_subject_tags: tagscategory
In your case, I think you'd want to copy ships to, e.g. shipscolumn, apply your transformation to shipscolumn, then use custom_column_ships to apply it to #yourshipcolumn.