Although I actually quite like the 2nd Edition of the Conan rules from Mongoose, I really cannot see why not.
The BIGGEST obstacle is the lack of any Healing Magics available to a party, thus rendering all Characters really vulnerable to an untimely end.
In the Conan RPG, they circumvent this with Fate points - but in the SEIGE system I would probably use a "Luck" Attribute in stead, and give it the same 3-18 range (and option to make Primary or not, after all some people are luckier than others) as other C&C Attributes.
Races are another point of contention, as the 3.5 based rules give racial bonuses in the +2/-2 range - whereas in C&C all bonuses are in the +1/-1 range. Its easy enough to correct, dropping +2/-2 to +1/-1 and (should the occur) reducing +1/-1 racial adjustments to zero's across the board. Don't forget there are NO Demi-Humans really running around in the Conan Universe, those "sub-humans" that do exist (like the Black Giants of the Western Seas from Savage Sword of Conan #22) are not really suitable for use as Player Characters.
Classes are (kinda) the easiest to accomodate, so here are some "rough" thoughts.
Borderers & Nomads
The quite happily come under the heading of Rangers, they are similar (but not the same) in many respects, I would "tweak" the Ranger class on a character by character basis if variations are required.
Nobles & Knights
I would recommend using the Noble Class from the C&C Freeport Companion. Knights (in the Core Conan Rulebook) come under the heading of "Noble" but luckily in C&C there is a seperate Knight Character Class.
As I have pointed out, Hit Points and Healing could potentially be a problem. In addition to having either Fate Points or a Luck Stat, there are a couple of other things you can do to make things easier.
At First Level, give your PC's Starting Hit Points equal to -
A Dice Roll (or Maximum) + Con Bonus + Constitution Score. It might seem a lot, but there is a rationale to it.
The People of Hyboria are a tough bunch, much tougher than the average FRP "Joe" - plus it increases their "durability" in an environment with limited Healing.
Rather than 1D3 Hit Points regained during the use of a characters Healing Skill, give them a Higher Dice Roll - or (which I prefer personally) give them 1D3+ the number the task was beaten by.
E.G. If the difficulty was 17 (the number of Hit Points lost in this case) and the character performing the healing rolled a 12 (after modifiers) the person receiving the aid would be healed 1D3+5 Hit Points of Damage. HOWEVER it would take 2 minutes of "tending" per Hit Point healed, and the injured character would then need to rest for a number of minutes equal to the number of Hit Points healed multiplied by 5 (so 6 points of damage healed would require 30 minutes of rest), if the healed character becomes active during that time, he would only receive half the number of Hit Points restored.
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