For fermented pickles, alum can be used safely to firm up the pickles. However, it isn't necessary and is often not included in tested recipes. Alum doesn't work to improve the firmness of quick-process pickles.
The calcium in lime will also enhance pickle firmness. You can use food-grade lime as a lime-water solution for soaking fresh cucumbers 12 to 24 hours before you pickle them. In order for the pickles to be safe, you'll need to remove the excess lime absorbed by the cucumbers. In order to do this, you'll need to drain the lime-water solution, rinse and then resoak the cucumbers in fresh water for one hour. Repeat this process two more times.
Another option for firming pickles is to process the cucumber for 30 minutes in water at 180 degrees Fahrenheit. This will also help to prevent spoilage, but only if the water temperature never drops below 180 degrees Fahrenheit.