No. I understand that it might be acceptable in Indian English, in which the continuous is much more common, but in most other variants of English, it should be "This item belongs to their company".
Verbs which indicate a state (instead of an action) do not normally use progressive tenses. They use simple ones. Thus, we do not say: I'm owning/knowing/owing ... etc.
Instead we say: I own/know/owe ... etc.