Yes the installer can keep the original function but add a button to the existing unit if he's thorough.

 

I had this installed on my '05 IS300 before I sold it a while back.

http://www.advancedkeys.com/index.htm

 

My mothers 06 GS430 has the push button start. Kind of surprised Lexus did not have it for the LS...

 

The 2006 GS was a new platform for that model which was designed with a push button system incorporated into it. The LS at the time was not designed with pushbutton tech in mind and it was in its last years with a all new model on the way. I'm assuming Lexus wasn't gonna waste time and money putting that tech in a model for its last year of life when the LS460 was already being produced and introduced.