“If they shall fall away” The Translators of the KJV

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In the KJVof 1769, for Hebrews 6:6 we read,

The word “If” here has been the subject of very much theological discussion, but where did the translators of the KJV get that word from?

The Greek text from which the KJV was translated reads,

kai parapesontas, palinanakainizein eis metanoian, anastaurountas heautois tonHuion tou Theou kai paradeigmatizontas. (transliterated, unaccented)

Compare the following translations:

-- New King James
if they fall away, to renew them again to repentance, since they crucify again for themselves the Son of God, and put Him to an open shame.

-- American Standard
Hebrews 6:6 and then fell away, it is impossible to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame.

-- Revised Standard
Hebrews 6:6 if they then commit apostasy, since they crucify the Son of God on their own account and hold him up to contempt.

-- International English
Hebrews 6:6 but they have fallen away. It is impossible to bring them back to a change of heart. In their lives, they nail the Son of God to the cross again, shaming him publicly.

-- New American Standard
Hebrews 6:6 and {then} have fallen away, it is impossible to renew them again to repentance, since they again crucify to themselves the Son of God and put Him to open shame.

-- New Jerusalem
Hebrews 6:6 and yet in spite of this have fallen away, it is impossible for them to be brought to the freshness of repentance a second time, since they are crucifying the Son of God again for themselves, and making a public exhibition of him.

-- New American
Hebrews 6:6 and then have fallen away, to bring them to repentance again, since they are recrucifying the Son of God for themselves and holding him up to contempt.

-- New Revised Standard
Hebrews 6:6 and then have fallen away, since on their own they are crucifying again the Son of God and are holding him up to contempt.

-- Young's Bible
Hebrews 6:6 and having fallen away, again to renew [them] to reformation, having crucified again to themselves the Son of God, and exposed to public shame.

-- Darby's Bible
Hebrews 6:6 and have fallen away, crucifying for themselves as they do the Son of God, and making a show of him.

-- New Living Translation
Hebrews 6:6 and who then turn away from God. It is impossible to bring such people to repentance again because they are nailing the Son of God to the cross again by rejecting him, holding him up to public shame.

-- International Standard Version
Hebrews 6:6 and who have fallen away, as long as they continue to crucify to themselves the Son of God and to expose him to public ridicule.

-- William's NewTestament
Hebrews 6:6 and then have fallen by the wayside --<d> it is impossible, I say, to keep on restoring them to their first<e> repentance, since they continue to crucify the Son of God to their detriment<f> and hold Him up to contempt.

Would any of you care to venture an answer to the question asked above in bold type?