Will this cause any issues in my openfire's message sending functionality?
Not sure, but you definitely should not do that. See also XEP-0134: XMPP Design Guidelines
Basically instead of
<message type='mytype'…>
…
</message>
you want
<message type='normal'…>
...
<myextension type='mytype'/>
</message>
Where type='normal' can also be any other type which message routing semantics suit your use-case.