I'll weigh in on this too. I think our original usage scenario with OCS was that people would use *it* to publish all conference-related content (which it can obviously do), but over the past while I've seen a number of organizations want to do the same as what you're describing: publish conference proceedings, but in journal format. (Actually, we're more or less doing this ourselves, although it's not necessarily automatic: presentations from the PKP Scholarly Publishing Conferences
are solicited for publication in the Journal of Scholarly Research and Publication
, although in both cases they go through peer review.)
I think the best way to go about doing this in the short-term is to use the article/paper import/export plugins, as Matt describes: export the conference proceedings into OCS-compliant XML; munge the XML so that it is OJS-compliant; import into OJS. It's that middle step that's tricky, and I'm not entirely sure off the top of my head exactly how tricky. I'll try and give it a couple of test runs myself though, and let you know what I learn.
Long term: I think the plugins should have an interoperability function built in. I'm not sure of the best way to do so, since the underlying publishing structure *is* so different (eg. OCS doesn't really do much with vol/issue/year demarcations). Given that the most typical scenario is OCS->OJSj rather than OJS->OCS, maybe it would suit to extend the OCS export plugin to export to OJS-compliant XML? This would still require some user interaction to define vol/issue/number etc. information, but that could happen either at the plugin level by request, or at the hand-editing XML level (with the plugin providing helpful blank elements to fill out or something).
Any thoughts on this possible solution? Super-long-term, of course, we'd like for one "PKP system" to be able to handle both types of publication content (which is a goal of our ongoing modularization effort); but we're not to close to that yet. If the above solution looks workable, I can submit a bug report; and if you have any time or ability to work on the problem of course, we'd be happy to help any way we can.