ESX exposes NFC (network file copy) as a service which Converter uses to perform conversions, and NFC uses the management network. AFAIK this can't be changed.
There is an exception, though - if you do Linux P2V, the cloning goes through the VM network and you won't have this issue.
I think you may have a more general problem with this setup, as NFC is used not only by Converter (e.g. SRM, VMotion, etc...). See this: Why is vMotion using the management network instead of the vMotion network? - frankdenneman.nl for something completely different but that might also cause an issue with this setup.
Regards,
Plamen