There used to be a suffix for it, but adverbs don't exist in Dovahzul quite like they do in English. The now correct way to express adverbs would be to say "with ____". For example "truthfully" would be "with truth" or "in truth". "Courageously" would be "with courage". Here is some more on adverbs.

And what about -ing?

"-ing" would be "-taas".