Do you mean the passcode pop-up keyboard that Apple provides when you first start the iPad and turned on via Settings? Cause there is no way to change that and use a different app to do the same function. That function is hard-baked right into the iPad and iOS. You can make the password a combination of numbers and letters by expanding it (again, in Settings), but that's the extent of the change you can do to it.
If you didn't mean that - could you expand your question a bit more? Thanks.
Marilyn