Maybe wait more than an hour.
Anyway yeah you can probably add more characters. If you are going to do something like simplified Chinese with lots of characters then it gets quite tricky on older systems, and even adding say 5 more characters is not something I would suggest a new hacker do for an old system or one with a font setup like that (it is easy enough on a DS where there are font formats you can reasonably add to, this is not that though). The GB/GBC does not have very much memory, does not have very large ROM sizes and trying to handle lots of characters actually represents something quite difficult to do on it.
GB/GBC hardware documentation
https://bgb.bircd.org/pandocs.htm
If there is a Japanese version of the ROM then start looking at that. They will usually already have options to cycle out character sets which you can use here, rather than trying to add that from scratch to a simple one like that, not to mention a few more characters to begin with.
Do you really need all the characters though? Looking at the game* there is an opening text crawl, some dungeon names, the usual energy, life and whatever (which even the Japanese has in English), some text for the second to last boss and some ending text. That is almost enough that you can probably cut it down to just what you need to say that (and maybe change what you need to say to mean the same thing but use fewer different characters) rather than having a whole copy of Xiandai Hanyu Cidian shoved in there.
*video
and Japanese version
ドラキュラ伝説 is the name (thought this would be ドラキュラ伝説II) if you need to search in Japanese.
This is my translation of the title
