Modular Addict has Sifam/Selco 3/03 T18 knobs for a good price:
https://modularaddict.com/selco303-knobs

If you just want to combine signals in the rack, then this is the most space efficient module I know:
https://www.clank.eu/sum
If you want anything more than that, like control over levels, then you need another mixer.

Zlob Entropy: S&H/T&H, white/pink/brown noise, smooth random, 3HP, $100, or less for DIY (easy through-hole soldering)
There is now an equally feature-packed expander that I havn't tried yet