Not exactly. They are .html files, but are
MARISA based.
Here's some information on the actual format Kobo uses:
https://pgaskin.net/dictutil/dicthtml/format.html
If your dictionary already works under koreader (i.e. is in stardict format), then you can use
pyglossary to convert it into Kobo format.
Here are some instructions on how to sideload a custom dictionary onto a Kobo:
https://pgaskin.net/dictutil/dicthtml/install.html
EDIT: Although on recent firmware versions, you can just place the custom dictionary into .kobo/custom-dict/ rather than having to edit the database first or replacing an existing dictionary like it says in the link above.