IMSI's web page listing file import/export capabilities for TurboCAD Windows hasn't been updated to 2019, yet (get off the pot and do that, okay?), so I can't confirm that STL import went away in this version.
However, if you can upload the file, one of us can convert it to TCW.
TC Pro Platinum 2020, 2019, 2018, 2017, 2016 & 2015 (all with LightWorks & RedSDK) & V21
System: i7-5820K @ 3.30GHz, ASRock X99 Extreme4, 16GB DDR4-2133 RAM, Gigabyte GTX 970, Samsung NVMe SSD 950 (256GB), Windows 7 Pro (64-bit) SP1