The hybrid modular multilevel converter (MMC) based on half-bridge sub-modules (HBSMs) and full-bridge sub-modules (FBSMs) can operate in low DC voltage and clear DC side fault current. However, the cost and power loss are much higher in hybrid converters. An auxiliary full-bridge converter (AFC) is designed to reconstruct the converter structure in the arm level, the HBSMs output capacitor voltage through the AFC can attain a similar capability to hybrid MMCs. The operation principle of the auxiliary full-bridge converter is discussed, the low voltage operation and non-blocking fault ride through control are verified in a two-terminal DC network simulation. By economic analysis, the power loss of AFC is similar to HBSM MMC but the total investment is lower than hybrid MMC, making the AFC a promising solution to improve existing HBSM converter with more controllability.