If Serleran's suggestions don't work for you, and you want to put some work in.....
Although it's intended for the Rules Cyclopedia, the article below might be a good place for you to start from:
http://www.tridrpg.org/characters/newclass.php
Obviously, if you use this article, you'll wind up with some differences between what the article costs things as, and how C&C costs things out. I've got 2 suggestions:
The first (and simplest) would be to work out how each of the four iconic classes (Fighter, Thief, Magic User, Cleric) cost out using Erin's system. If you want to get another one to use as a baseline, go ahead and build the Ranger as well.
Once you've built the C&C classes using Erin's article, compare how the XP requirements look between both Erin's article, and the C&C version. Divide the C&C cost by Erin's cost, and you'll have a number. That number is what you should use to multiply times the XP cost Erin gives, in order to find out what the "actual" cost would be under C&C.
For example, say you build the Fighter using Erin's system, and it works out to 800 XP. You look over at C&C and see that it winds up costing 1000. So you divide your 1000 by 800 and wind up with 1.25 as the answer. Now you can turn around and build whatever Fighter-like classes you want using Erin's system, multiply the end result by 1.25 and you'll know it's roughly at the cost it should be for C&C. Follow C&C's approach to fill in the XP requirements for those levels that don't have anything special being given.
Go ahead and do the same for the remaining 3 classes (4 if you use the Ranger), and you're basically set. Any particular class you want to build/convert is going to fall broadly under one of those types of characters, in terms of it's fighting capability, skills, and casting.
The second possiblity is more involved.
Basically you'll do something similar to the above, only you'll be converting _all_ the C&C classes. Don't bother doing the Perfect Class / C&C XP comparison yet. When you've finished rebuilding them all, you get to make a decision. You can do the XP comparison and division for all the classes, and then apply the most appropriate factor to the new classes you build, or you can average them out, and apply that new averaged factor to _all the classes. The ones you've just rebuilt, as well as the new ones.
The point to averaging the factor and then using that for the old classes as well as the new, is to essentially put everything on the same rough level in terms of cost. It means that the XP level progression charts printed in the books will be off, but it will also mean that your new class (or classes) will be much more likely to be "equally balanced".
If you're looking to go in a different direction, there's a pdf product called Buy the Numbers that is basically a deconstruction/class building type of system for 3.x. You could basically follow the same essential steps I outlined above. Buy the Numbers also provides a cost breakdown of the d20 fantasy classes, and what the "real" XP cost for them would be; as opposed to the single XP track that they all use.
The other alternative system for 3.x to C&C that you could use is the Anime SRD. It's free, unlike Buy the Numbers, and can be downloaded from here:
http://www.guardiansorder.com/games/d20/srd/ It provides a breakdown of the d20 fantasy classes as well, although it's not quite as detailed.
Whichever approach you take, there's going to be some work involved.
A pretty big disclaimer: at some point the CKG is going to be published, and at least in theory it should offer a solution (or perhaps more). So doing something like this is going to be a fair amount of work that might be completely invalidated to a greater or lesser degree once it is published. On the other hand, it might be groovy enough for people to use anyway, assuming you put it out for public consumption, or you might prefer your approach to the CKG. Only time will tell.
With the exception of the Deconstruction document that Serl made and subsequently pulled, I haven't seen any system publically put forward specifically for building C&C classes. I've done some rough work on something using Buy the Numbers, but it's not yet in a state where I'd consider making it publically available. Serleran's document, the thing I'm using, what the CKG eventually publishes, the Yggsburgh thing, all of it is based on the what the author feels is "balanced" for the kind of games they like to run. You'll just have to decide what works for you, and take the plunge.
Or go the easy route, and follow Serleran's suggestions.