I seem to recall that back in Foundstone 6.5 it had a capability to merge scans together using FSDBUtil, this seems to have been removed in MVM 7.0, - is there a way to do this as whilst Asset based reporting is powerful it doesn't have all features that scan reporting does (ability to suppress ignored vulns, network map etc.).
They removed this option from the tool when Asset Reporting became possible. I see your point that it's not quite the same, but there isn't any plans to add that feature back into the tool, nor is there any other way to accomplish what you're asking for.
The best method would be to have something built into the product to do this vs. having to have a separate tool doing it. have you considered submitting a Product Enhancement Request?