And you just did that with your parenthetical (if it was possible). If it were not possible, the author would have never given it as a conditional.
The passage begins "for impossible" not "if possible". What does he say is impossible? For those enlightened, tasted the heavenly gift, partakers of the Holy Spirit, tasted the good word of God, and powers of the world to come. IOW the writer is assuring us of salvation telling us what is impossible, and in the next verse he tells us why it is not possible. Because IF they fall away they could never be renewed unto repentance, because that would mean Christ must be crucified and put to open shame again!
Hebrews 6:4 (KJV) For
it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost,
Hebrews 6:5-6 (KJV) And have tasted the good word of God, and the powers of the world to come, If they shall fall away, to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put
him to an open shame.
The final assurance of our salvation, possession of eternal life is hearing all these things were said that we might have confidence, knowing that the life we receive when we are born again is certain and can never be lost. Though we are warned because of the tares among the wheat we have those things that accompany salvation FOREVER!
Hebrews 6:9 (KJV) But, beloved, we are persuaded better things of you, and things that accompany salvation, though we thus speak.