It is precisely because Catholics have a high view of sacred Scripture that we believe in infallible magisterial documents without making them additional Scripture. Simply being infallible and binding does not make a document equal to Scripture, because Scripture is not simply infallible and binding: It is the inspired Word of God, divine revelation which has God as its principal author. Magisterial documents, even the most solemn and authoritative, are purely human writings, not divine; they are about divine revelation, but are not themselves revealed. God is not their author, only their editor; he does not inscribe truth into them, but only keeps error out. The notion that any inerrant document amounts to "additional Scripture" betrays a shockingly low view of Scripture, one which defines Scripture in purely negative terms as that which is errorless.
Beyond this, the very charge of "additional Scripture" presupposes that the writing of Scripture has ended, a doctrine affirmed by the authoritative teaching of the Church, but nowhere made explicit in sacred Scripture. Revelation 22:18 does not refer to writing additional Scripture, but to tampering with "the words of this scroll," the Book of Revelation itself. Similar warnings can be found in the Old Testament, long before the end of the age of revelation. Ironically, this charge presupposes a doctrine that is explicitly taught only by the authority of the Church!