[NTG-context] Automatic sorting of german umlauts in registers

Hi! Is there a way to sort german umlauts in registers automatically like the DUDEN does? For instance the DUDEN treats the ü like an u rather then ue. A little example: Huefte Huf Hüfte Hut The workaround with xindy is no good idea, because it removes the references in the pdf. Regards, Eyke