Yes, you can add PVA (Polyvinyl Alcohol) fibers to an existing mix, such as concrete or mortar, to improve its properties. PVA fibers are commonly used as a reinforcement material in construction to enhance the durability, strength, and crack resistance of the mixture. These fibers can help control cracking caused by shrinkage and improve the overall performance of the material.

When adding PVA fibers to your existing mix, here are some steps to consider:

1. Mix Compatibility: Ensure that PVA fibers are compatible with the existing mix. It's a good idea to consult the manufacturer's guidelines and conduct some small-scale tests before adding them to a larger batch.

2. Fiber Length: Different applications may require different fiber lengths. PVA fibers are available in various lengths, so choose a length that suits your project's requirements.

3. Mix Design Adjustment: Adding fibers to the mix can affect its consistency and workability. You might need to adjust the water content or other components of the mix to maintain the desired consistency.

4. Mixing Procedure: Add the PVA fibers to the mix during the mixing process. This can be done using conventional mixing equipment. Make sure the fibers are evenly distributed throughout the mixture.

5. Adherence to Guidelines: Follow the manufacturer's recommendations for the correct dosage of PVA fibers. Adding too many fibers can lead to clumping or affect the properties of the mixture.

6. Testing: Once the fibers are added, it's a good practice to test the mixture's performance, such as its strength, crack resistance, and other relevant properties. This will help ensure that the addition of PVA fibers has the desired effect.

7. Application: Apply the mixture as intended, whether it's for casting, plastering, or any other construction purpose. Keep in mind that the presence of fibers might slightly alter the application process.

8. Curing: Proper curing is essential to achieve the desired properties in the finished product. Follow the curing recommendations for the specific mixture you're using.

PVA fibers are just one type of fiber reinforcement. Other types of fibers, such as steel fibers, synthetic macro fibers, and natural fibers, also have their own advantages and applications. The choice of fiber type depends on your project's requirements and the specific properties you aim to enhance.