Not sure what you are looking for. RoboHelp has always been
an application that gave no consideration for backwards
compatibility to create something in a newer version that would
work in an older version. This doesn't mean it cannot happen, only
that RoboHelp doesn't facilitate it.
As long as you aren't using features that aren't supported in
the older version, you might be okay. For example, Snippets were
new to version 7. Version 6 didn't support them. So you *COULD*
theoretically open a version 7 project in version 6 by first
generating .CHM output. Then renaming or deleting the .XPJ and .CPD
files. Then pointing at the .HHP file. If you did that, RoboHelp
would likely re-create the missing files and off you would go.
Keep in mind that I've not tested this actual combo, so it
may or may not work for you.