yep. I believe there are several posts in the archives here that talk about how to do that. There are differences among the various silca models, but basically,
1. remove the plunger
2. cut barrel to the correct length.
3. gently work out the plug with the leather washer that fits into the plunger. Don't break it or you can chuck the whole mess.
4. cut the plunger to the correct length.
5. Place the plug with the leather washer back into the cut plunger.
6. peen the side of the plunger to hold the plug in place.
7. chamfer the inside of the barrel where you cut it, a little. This will provide the cap a starting point to
thread itself back into the barrel.
8. Thread it back into the barrel. I like to reinforce the new threaded area with a little piece of strapping tape to keep the barrel from splitting.

A tiny key is the phrase "the correct length". Don't go guessing, or you'll end up making a pump for your significant other's bike. really. this is the voice of experience speaking. more than one time.