Ringdroid should let you open up the default ringtones in the list and save it to a new name... It should show up on your SD card after that... Copy that over to your computer, and once you do that, you can insert a silence using any audio recoding software... I suggest audacity as it's free... You can insert any amount of time, save the file back and now use ringdroid to assign it as default ringtone or preferred ringtone on any given name...
I hope that was clear...
It's a couple of steps, but well worth the end result if that's what you're looking to do...