When they are confronted with the overwhelming evidence that απαραβατος in Heb 7:24 does not mean "untransferrable," the more intellectually honest Evangelical Protestants (few and far between as they are) of the LDS Church will instead argue that the rest of the verse precludes anyone else holding the Melchizedek Priesthood as, they claim, only those who offer and present their propitiatory sacrifice before the Father can hold the Melchizedek Priesthood. This was ably answered by someone who posts as "MormonMason" on the Mormon Dialogue and discussion board (emphasis added)
Here is the thing about that passage in Hebrews 7, which is so very often translated incorrectly. The passage refers rather to the office of High Priest in its functions. Another problem is that people assign to the Greek word aparabatos an active sense that nowhere appears in any document in which it is used; and it has been used far more often than the lone instance that occurs in the New Testament. The only way it could mean that it indicates something "not transferable" is if it holds an active sense.
But, the evidence of usage shows that it did not appear to have such an active sense as used to be claimed for it in the old literature before the papyri containing the word were found. The standard Greek-English Lexica point out this little fact that continues to be ignored in the evangelical literature for the most part.
But, to return to the point I made about the specific office, there is one thing in which Christ cannot be succeeded, and that is the sacrifice which he made for sins, such as the High Priests (Aaronic) of Israel did. However, that did not change the fact that there were others who held the same priesthood. They just were not the functioning High Priest for the years in which they did not serve. But, there still were a number of others who held the Aaronic Priesthood.
It is the same with the Melchizedek Priesthood. There is only one who holds a specific responsiblity as part of his office to offer sacrifices for sin, and that is Christ. However, that does not preclude the existence of others who hold the same priesthood and office. They just do not function in the same capacity as does Christ, who sacrificed himself once for sin.
That is how I see the meanings of the passage in context.
There is no logical or exegetical strength to such an argument against Latter-day Saint claims concerning the Melchizedek Priesthood.