Most decompression/compression programs support extracting/adding individual files from/to an archive. This should work, but I only tested it against a .7z file. But I am not aware of any reason why it would not work with .rar files.

7z.exe e <archive_name.rar> <path_to_file_in_archive>
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Here's an example. Given a 7-zip file called test.7z, containing:

test
test/foo
test/bar
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This should extract just 'bar' to your current working directory:

7z.exe e test.7z test/bar
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