I don't see this feature documented anywhere in OpenNMS, but from the wiki:
If a particular outage calendar is included in a poller package, then polling will not occur during this time. Note that this does not mean that the service will be considered "up" during this time. If the maintenance is started a minute too soon and an outage is detected, then no poll will be made to restore the service until after the outage window has closed.
So, the essential effect of a scheduled outage is to cease polling during that interval, so that the actual outage (caused by planned maintenance) is not recorded.
So, to mimic this, you could retroactively delete the entries logged by the poller during a scheduled maintenance that was not included in OpenNMS. This seems to require creating SQL queries to modify the events database. See event maintenance for more information on how to specifically accomplish this goal, as the specific answer will be dependent on specifically the items you wish to delete.