Yes, if the maps are out-of-copyright then it is obviously no problem using those maps for OSM.
Best practice dictates that you add a ‘source=’ tag to all the data that is uploaded from sources like yours. e.g. ‘source=out of copyright map military map’. This allows, if problems arise, that the data from this source can easily be differentiated from the other data.
The out-of-copyright wiki page gives you hints about converting old maps into OSM data. Additionally to that page, maybe you can take a look at GPSMapEdit as a converting tool.