Both the .indd and .idml files are produced in Adobe InDesign and it is only this program that can actually resave indd to idml files, i.e. achieve indd to idml conversion. This is most likely needed for the purpose of opening newer InDesign files in previous versions of the program. It appears that no 3rd party software is capable to convert indd to idml, so the original InDesign is your only option.
Open INDD file Open IDML file