In older versions of the software (specifically version 3.1), users could often load a library tooth or attachment in Expert Mode , hide other elements, and then use the "Save visible elements as STL" function.
To give you the most accurate steps for your specific situation, could you let me know:
If you are combining a custom attachment with a dental rim, you may need to use Meshmixer to merge the files to make them a single, watertight STL for printing.