Both the .indd and .idml files originate from Adobe InDesign and it is very likely that only this program can work with them and possible save one to another, i.e. achieve indd to idml conversion, most likely as an attempt to open newer InDesign files in older versions of the program.
Unfortunately, we haven't found other information about any other possible way to convert indd to idml, so using InDesign appears to be the only viable choice.