Unless someone's willing to take up the task of writing code to convert from plain text to HTML (probably via user-defined program which is a relatively significant undertaking) or you learn to program in python and do it yourself, converting from plain text directly to custom columns is easier.
e.g.
	Code:
	Search field:
#your_source_metadata_column
Search for:
Category: ([^\n]+)
Replace with:
\1
Destination field:
#fandom
 
	Code:
	Search field:
#your_source_metadata_column
Search for:
Story URL: https://archiveofourown.org/works/([0-9]+)
Replace with:
\1
Destination field:
#storyId