This is one of the extra files @catty-cb has maintained on her site and now has kindly passed over to us here at Simtropolis for hosting. Inside the zip is the LandmarkConverter.exe file.  Place it in your C:\GOG Games\SimCity 3000 Unlimited\Apps folder. There is also a Readme_LandmarkConverter.txt file in the zip which explains how to use the tool. (Tho it's really easy so just refer to that if something goes goofy.) I've used @Yohane Architects's Golden Coast Apartments building as the sample in my pics.   You will notice this file is not my creation and credit for keeping it alive goes to catty. This STEX entry will stay under my name tho since I'm in charge of the new SimCity 3000 Sub-Forum and I'll be the one to reply to comments or questions.