There is no way to designate an NPC as a Housing Merchant or an Escrow Merchant. I had tried to work around this when adding the housing merchants to Southern Stonepine Ridge chunk by designating them as Global Exchange and Banker NPCs (since they offer the services for these things as well), and, on a related note, neither Global Exchange nor Banker appear to be listed in the NPCs by chunk.
We should probably add these two categories to the NPC by chunk list, particularly since knowing where the Housing/Escrow merchants are in housing chunks is one of the things I know I am always trying to look up! :)