Can I import existing Lego kits into LEGO Digital Designer? Is there any way to import existing LEGO kits into LDD (LEGO Digital Designer)?

For example, I like to have this set in the LDD and 
https://shop.lego.com/en-US/Compact-Tracked-Loader-42032

I know I can download the PDF instructions, and I like to have the model in LDD or any other LEGO 3D designing tool.
The only way to achieve that is to manually build the set in LDD. If you don't want to do it yourself, check the Eurobricks library for sets built in LDD by the LEGO fan community. You can find virtually any set released within the last several years: https://www.eurobricks.com/forum/index.php?/forums/topic/41226-key-topic-official-lego-sets-made-in-ldd/

The set you mentioned is on the list, so all you have to do is download the LDD file. :)