Possible to prepopulate an inline entity form multiple with a url parameter?
Please try with Entity reference autofill and Entityreference prepopulate Modules. 


Add an entity reference field in your entityform.
The entity reference autofill will automatically fill reference value into field. We can load default value from URL. (?field_name = value)
Entityreference prepopulate will prepopulate the values. (you have to add existing fields for that.)


For more details please refer project page.

Another approach is,


  If you want to do this by coding, Just pass nid through url and use node_load() to load that node and create an entity form submission. If you want to create entity form with node creation use hook_node_insert or hook_node_presave.


Hope this will help you.