Custom Feature Shape

   Hello. I already experimented with the new Feature element and i also have read the wiki on the subject, but one thing i could not find anywere is Custom feature shapes. I thought we could use structure blocks and import it somewhere in there but everything i found was the Features tab in the editor that has many options, but not what i need. It has to be possible somehow right? I mean, in minecraft we have like... The desert well and stuff right? So, i am asking if anyone knows of a way to make a custom shape for the feature
 Thx -Rono