I guess that this would be pretty hard to impelement but I will look into it.
This feature actually already exists, press and hold GPS position icon in top right corner to enable it.
At the moment landmarks are imported from file "/home/user/MyDocs/.mapsi/import.gpx" and exported to ".../.mapsi/export.gpx". Supported import formats are Gpx 1.1, Kml, Kmz and Lmx. I will add possibility to select import/export file maybe in the next version.
I don't think that there are any download limitations. In the settings menu the icon with two arrows can be used to enable/disable use of WMS(Web Map Service). If you have WMS disabled map tiles won't be downloaded. You can also try to change used WMS server by pressing and holding that WMS icon. (Paikkatietoikkuna has 4 WMS servers)