The finnish NLS topo maps at kansalaisen.karttapaikka.fi use finnish KKJ coordinate system, so MaemoMaper would have to do on-the-fly warping of the tiles. Perhaps MapCruncher is the way. Here is quite detailed description of the KKJ coords (you can extract also some conversion code): http://www.viestikallio.fi/tools/kkj-wgs84.php#KKJ-PI
BTW, stitching can be automated by e.g. montage utility from the ImageMagick suite.