Hello,
being a bit pedantic here: the bulk import tool allows to import any kind of a file, so XML from another ECM platform is not a problem.
BUT, you are definitely talking about not just importing but also evaluating the file to recreate the logical structures from the old ECM system within Alfresco. This is something that the bulk import tool almost most likely does not support. This would require some form of customization / development to properly process the XML and create the appropriate (maybe not identical) structure based on Alfresco concepts.
What is transferable fully depends on what can be reasonably extracted from the XML. If it is a full export, I'd assume everything is transferable as long as there is an Alfresco concept that is similar in nature to the correspondong concept of the source ECM.
Regards
Axel