Yes, you can upgrade belts in d2. However, the process is a bit complicated and requires some specific items.
First, you'll need a Belt of the Titans. This belt can be found in the game world or purchased from vendors. Once you have this belt, take it to the Cube and use the following recipe:
Belt of the Titans + 2 Perfect Square Rubies + 1 Forgotten Soul = Upgraded Belt
The result will be an upgraded belt with increased stats. Keep in mind that this process is irreversible, so make sure you're happy with the results before proceeding.