The best way is to use the XML import tool (available either on command line in tools/importExport.php, or in the Journal Manager's "Import/Export Data" interface). See plugins/importexport/native/native.dtd for the DTD.
Alternately, if you don't have XML experience, you can go through the regular author submission process. The minimum steps required are described at http://pkp.sfu.ca/support/forum/viewtopic.php?t=580
Note that OJS 2.1, which will be released very soon, gives editors an option to "expedite" submissions -- OJS will automatically follow the steps described in the link above.
Open Journal Systems Team