Something

<!--* try.dtd *-->

<!ELEMENT foot (sock)>
<!ELEMENT sock (#PCDATA)>

<!--* wrapper.xml *-->


<!DOCTYPE wrapper SYSTEM "try.dtd" [
    <!ENTITY theDoc SYSTEM "try.xml">
    <!ELEMENT wrapper (foot)>
]>
<wrapper>&theDoc;</wrapper>


<!--* try.xml *-->
<?xml version="1.0" encoding="UTF-8"?>
<foot><sock>black</sock></foot>

<!--* valdiating *-->
$ xmllint --valid --loaddtd --noent wrapper.xml 
<?xml version="1.0"?>
<!DOCTYPE wrapper SYSTEM "try.dtd" [
<!ENTITY theDoc SYSTEM "try.xml">
<!ELEMENT wrapper (foot)>
]>
<wrapper>
<foot><sock>black</sock></foot>
</wrapper>
$