Yes, provided you use up-to-date SGML software which knows about the WebSGML Adaptations to ISO 8879 (the features needed to support XML, such as the variant form for
EMPTY elements; some aspects of the SGML Declaration such as
NAMECASE GENERAL NO; multiple attribute token list declarations, etc).
An alternative is to use an SGML DTD to let you create a fully-normalised SGML file, but one which does not use empty elements; and then remove the DocType Declaration so it becomes a well-formed DTDless XML file.
Most SGML tools now handle XML files well, and provide an option switch between the two standards. (see the pointers in the question on software).