I'm unaware of any "outside texts" that Protestants would consider equal to or adding to Scripture.
... adding to which scripture? Which bible? Which canon? The 1611 King James Bible perhaps?
It contained the Apocrypha. Do you accept the apocryphal writings as Scripture?
Irenaeus (130 A.D.) did not include Hebrews, James, 2 Peter, 2 John, 3 John, and Revelation in his list.
Maybe you accept Athanasius' list (296 A.D.), identical to the list in your modern New Testament, a list which he called "wells of salvation" and instructed the churches to bring in any writings in his list which they did not have, and take out any writings which were not included. Ah, maybe God inspired Athanasius to choose the true "canon". Yet, interestingly enough, Athanasius included Baruch in his Old Testament list. Should it be included as Scripture, or did Athanasius blow it? If so, then did God really inspire him to choose the correct writings to include?
But it seems Athanasius didn't settle even the New Testament. Codex Claromontanus (400 A.D.) included the Shepherd of Hermas, the Apocalypse of Peter, and the Letter of Barnabus.
We can't conclude that the list which God allowed to remain in our day is the complete "Word of God", the only writings which are "Scripture", flawless, without error. For He also allowed the Catholic list to endure which includes the Apocrypha, (of the "Deutrocanonicals" as Catholics call them).
So how do we know our Bible contains the correct list of "Scriptural books"? Upon what basis have we accepted this list?