Hello
As far I know, in order to open a custome window for a specific NPC, you must edit interface.u and create a script for it. Also if you want to modify this U file, you need to decrypt it, and have some knowledge on Unrealscript to make the script works for this NPC.
I know someone in this forum shared the compiler for interface.u but I think it was for IL, I do not know if it was for H5, you might search on RU forums where it was shared as well.
Can you tell us for which client are you planing to mod?
Cheers