You can use the ConvertExtension to export just that one file from the first repository into a new temporary repository, then use hg pull -f to import the new repository into the target repository.
Create a filemap for the ConvertExtension with the single line:
include path/to/file
Then use:
hg convert path/to/original path/to/temporary --filemap filemap
to create the temporary repository. Next, in the target repository, do:
hg pull -f path/to/temporary
to pull in that file with its history. This will create a new head, so use hg merge to merge it with the head in your target repository.