If you manage to export your data into a GPX file, you can import it into Mapy.com simply by dragging the GPX file onto the Mapy.com web page. After that, you can save it into "My Maps" — you need to be signed in to your Mapy.com account. You will then find the imported data under "My Maps" → "Saved", where you can also organize it into folders.

However, in this particular case, only points of interest (your saved places / map positions) can be imported this way, specifically if they are stored in the GPX file as <wpt> waypoints.

It is also possible to import <trk> tracks, for example recorded routes, if you have any.

Plans or other entities (such as custom areas) cannot be transferred this way, because each map service handles those differently and there is no universal transfer format for them.