It's actually complete according to my research, the released mangas are:
1. Under Grand Hotel (main story)
2. Kiss & Tell (spin-off Vol.1)
3. Last Dinner (spin-off Vol.2) - this is the vol 2 you're looking for, the one that goes right after Kiss & Tell storyline. You'll see that the beginning of this & the end of Kiss & Tell match perfectly
4. UGH Crossover (current one)
It's the Last Dinner that I'm not finding online. I'd like to read a translated version of that but I can't find it. Thanks for the list btw!
http://www.mangago.zone/read-manga/under_grand_hotel_kiss_kill/
Last Dinner starts at Vol 2 Ch 1
Ahhh...one eternity later...thank you..(⊙…⊙ )
This sequel would probably make a whole lot more sense if UGH - Kiss and Kill with Norman, Erie and Mutou had been fully translated. I'm trying super hard to follow the damn story based on what I remember from Kiss and Kill. Lets all pray to the Translation Gods to complete Volume 2 of Under Grand Hotel: Kiss and Kill. ┑(￣Д ￣)┍