One of the main reasons I went with SiG is the fact there is NO external safety. Nothing else to get caught on clothing and NO extra step before engaging. It may sound stupid or unsafe to some, but you have to cycle the trigger before it will go bang. The design of the internal safety features is what got me interested in them and the fit and feel got me hooked. YMMV, but I have absolutely NO issues with not having an external safety mucking things up.
Perhaps a matter of preferrence or training, but it's really a non-safety issue as I carry mine chambered all the time. Always have.
Specifically abbout the retrofit, I'm not sure but I lean toward the probably not side. More info may be found on Dr. Lunde's site: http://lundestudio.com/SigProFAQ/ or you may contact SiG and ask. Installing a manual safety isn't as simple as changing a Classic "P" series from DA/SA to DAO or vice versa. There is milling involved to the slide itself, not to mention the SP2009 has been out of production for quite some time.