First, a caveat: I used the HMVC extension, but not the ORM Datamapper.
As far as I can see, the two extensions have separate goals. In principle, I do not see a conflict.
The HMVC extension is useful when your view consists of several subviews. This allows you to modulate your application so that your views can be built out of several controller actions.
Datamapper ORM PHP . SQL- . , , . Datamapper CRUD .
HMVC , . Datamapper ORM , . , Datamapper , HMVC .