Novak makes a "bar-dot" set with a horizontal bar in the rear sight that's probably the closest you'll come to mirroring your current sight set up. I've been underwhelmed with the fit of Novaks on Sigs, though; the flat bottom of the sight doesn't blend particularly well with the rounded Sig slide. Additionally, when I was looking into this option for my P226, Novak actually recommended against this particular sight, saying it tends to loose it's "glow" faster than a traditional two-dot rear.
FWIW, lots of very experience gun guys swear by only having a glowing dot on the front sight. I am getting to agree with them as my eyes get older, and it becomes harder to pick up and focus on the front sight when it's the furthest away of three equally bright dots of the same color... You might want to just get a good Meprolight sight for the front (they have a very clear, bright outline around the vial for daylight shooting) and leaving your current rear sight as is.
Regards,
Kevin