Unfortunately you cannot currently. The files that are in the local copy are
recorded in the database, not looked for on disk. 

There is a Current Feature request to add this ability "Download web site(s) +
Use existing assets (Action)" #113 but this not yet made it into the program.

Here is the link
<https://github.com/xroche/httrack/issues/113>

Posting to the request might help it get more consideration.