|Convert or else!|
Duke Omote's Advanced Castles & Crusades Page - This page has all kind of rules expansions that incorporate rules from various D&D editions into C&C. For example, Omote has rules for Skills and Talents (a.k.a. Feats) that might make converting PCs easier. If you are looking to convert older D&D to C&C, he has rules for Racial Classes and more.
The Crusader's Companion - Peter J. Schroeder compiled a ton of great gamer-created C&C rules into this impressive tome and it is just awesome. It includes a lot of d20 SRD monsters, items, spells, etc. that aren't in the C&C core rules. Also, the Castle Keepering chapter has a Conversion section that provides tables and charts to help you convert D&D materials from other editions into C&C.
The Castle Keepers Guide - While it doesn't deal with conversion specifically, it does include rules for Skills and Advantages (aka Feats) that might make converting 3.0 or 3.5 D&D to C&C easier.