Yes, this is allowed, but combining items is generally more expensive than making a separate item
The rules for creating magic items in Pathfinder allow you to improve an existing magic item with new abilities:
Sometimes the lack of funds or time makes it impossible for a magical craftsman to create the desired item from scratch. Fortunately, it is possible to improve or rely on an existing magic object. Only the time, gold, and various prerequisites required for the new ability to be added to the magic item restrict the type of additional powers that can be placed.
The cost to add additional abilities to an item is the same as if the item were not magic, minus the value of the original item. Thus, a +1 long sword can be transformed into a +2 vorpal long sword, the cost of its creation being equal to that of a +2 vorpal sword minus the cost of a +1 long sword.
If the item is an item that occupies a specific place on a character's body, the cost of adding any additional ability to that item increases by 50%. For example, if a character adds the power to grant invisibility to their +2 protection ring, the cost of adding this ability is the same as creating an invisibility ring multiplied by 1.5.
Adding more magic to an existing element can be quite simple or very mathematical. If the item's current and proposed abilities follow normal pricing rules (especially with weapons, armor, and shields), adding the new abilities is to subtract the old price from the new price and how many days it takes to make a difference.
For most other items, GMs should use the different abilities rule to determine the item's new price: increase the cost of the new ability by 50%, add it to the total price of the item. object to get the new price. Then subtract the old price from the new price to determine the difference and determine how many manufacturing days it takes to cover the difference.
In this case, we are combining different capacities rather than simply improving an existing capacity, so by following these guidelines, the base price of adding the properties of one +2 belt of incredible dexterity has a heavy belt is 6,000 gp (4,000 gp * 1.5), and the resulting element would be worth a total of 8,000 gp. As the price difference is 6,000 gp, the craftsman performing the upgrade will need 3,000 gp of materials and 6 working days, just as if he were creating a separate item with a base price of 6,000 gp (and, of course, it must meet all the prerequisites for developing a belt of incredible dexterity as usual). If you need to hire an NPC to do the job, they should probably charge for what they would normally do to make a 6000gp item from scratch.
However, you can note that if they did it the other way around – by adding the properties of a heavy belt to an existing belt of incredible dexterity – these guidelines suggest a slightly cheaper final price: 4,000 gp + 1.5 * 2,000 gp = 7,000 gp. A generous DM could allow the use of the cheapest order to determine the final price, and then the price difference is only 5,000 gp (7,000 gp – 2,000 gp).