i think you'll have to use RulesetConverter.exe
browse though its RulesetConverter.txt and stuff and i can give a hand if req'd.
https://github.com/kevL/OpenXCOM.Tools/blob/master/RulesetConverter/RulesetConverter.txtnote: I successfully used the RulesetConverter on Piratez and loaded its maps several months ago ...
edit
that's complicated so i'll try to rephrase it. Hunt around in your Piratez directories for the ruleset (.RUL) that contains its terrains. There may be more than one. but may be not.
Leave MapView's Configuration as is -- that is, configured to open Maps in your standard Xcom installation dir. (or wherever if you'd prefer to edit files in a /scratch directory, instead of making backups)
Once you've found a Piratez ruleset that contains terrains, run RulesetConverter.exe
browse to the ruleset
choose GameType UFO
click add Basepath
browse to the *parent folder* of the Piratez MAPS/ROUTES/TERRAIN directories
Click Convert and a MapTilesets.tpl file ought appear in the RulesetConverter's current directory.
Then open it in a texteditor like Notepad++
Also open the MapTilesets.yml file that's in the subfolder /settings of MapView.
Copy the stuff in the .TPL file into your MapTilesets.yml file, obeying the laws of
thermodynamics YAML.
Save it and try running MapView
if you find more than one ruleset that you'd like to import, Rinse & Repeat...
ps. That's off the cuff of my head, play around with it and i'll give advice if ya want,