I am not shure, that Lego designer has special tool for it.
1. For creating parts they use some cad software(i almost shure Autodesk), probably with few their own plugins.
2. Designers create models from real bricks
3. For creating instructions they use some their own software, that, i think use meshes from ther parts devvelopment team, but i think, they are not optimised for tool as LDD.
So there is no
for common users, all their software is only fo internal usage.
And adding parts to LDD, creating models for Lego games is hard and long process.
1. You need to optimase shape for it.
2. Reduce amount of polygons(ldd use 12 for circle,lego star wars 8)
3. Add connection data.
And i think it if they abandoned LDD, it is dead, because there will be no way to add parts to it without tons of problems.