Peeking over the edge of your bucket... it looks like you could slip the rootball out, add about 6 inches of soil, and replace rootball. Make sure the rootball is dryperhaps do this before your next scheduled watering. (wet soils fall apart) Measure first. If you have 6 inches of room at the top, add 4 or so inches of medium (with extra perlite if possible) under the rootball. Replace and water. Be sure to leave an inch or two at the top for watering.

Done right, you can keep a plant in the growth stage for years.

The longer you let them grow, the higher their requirements will be for flowering. Rootbound (restricted nutrient and moisture uptake) and light issues (stretching, mid and lower leaf neckrosis) might pose a problem if you let them continue too much longer.