There is no automatic way for Opus (or anything else) to know the HTTP URL of a file on an FTP site. The site's web server (if it's even running one) could put the file at any URL in the world...
For sites where the FTP -> HTTP rule is simple you can do it using a regular expression, which the command I gave can take.
For example, to replace "ftp://ftp." with "http://www." in the URL:
Clipboard COPYNAMES=url REGEXP ftp://ftp\.(.*) http://www.\1For anything more complex, you could use VBScript (maybe inside of an Opus rename script like this example).