That design has some odd aspects. -These days it's usual to use op-amps in a virtual-earth application. That design has a fader between the summing point and the gain stage... basically an all-passive front end with a simple make-up gain stage... noisy and prone to crosstalk/bleed. -It's essentially how I built the first mixer that I ever designed and built myself, and from that design I learned the pitfalls... I'd never do it that way again!
You could just keep adding more passive inputs on the front end, but they will interact noticeably due to the lack of a virtual ground at the summing point.
-Not how I'd do it at all...