Resolution:
No. Nobody other than W3C standard committee, has the authority to modify the standard schemas, including the "envelope" schema.  
The schema itself: http://schemas.xmlsoap.org/soap/envelope/
says:
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This document is governed by the W3C Software License [1] as described in the FAQ [2].
[1] http://www.w3.org/Consortium/Legal/copyright-software-19980720
[2] http://www.w3.org/Consortium/Legal/IPR-FAQ-20000620.html#DTD
By obtaining, using and/or copying this work, you (the licensee) agree that you have read, understood, and will comply with the following terms and conditions:
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From [2] W3C Intellectual Property FAQ:
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5.10 Are schema or document definitions covered by the document or software terms?
Check the actual schema or DTDs for a specific license.

Otherwise, while schemas and DTDs are frequently part of our specifications and seemingly fall under the document copyright terms, you may use them under the W3C Software License. The software license requires that you include/retain the copyright attribution and that you indicate any modification that you make. In the case of DTDs and schema, you must indicate your change by changing the namespace and/or public identifier so to distinguish your altered version from the W3C version. We further appreciate a couple sentences regarding who made the modifications, when, and what changes were made — a common software documentation practice.
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So when you modify/extend the "envelope" schema, you must change the namespace to distinguish your altered version from the W3C version.

Can you modify/extend W3C standard SOAP "envelope" schema in WSDL file?