I need a phone splitter (RJ11) to split 1 signal into 4 or more outputs. That's no problem, those can be bought in any store. But I was wondering if I can use a RJ45 cat6 patch-panel?
RJ11, RJ14, and RJ45 are connector types. Cat6 is a cable standard, indicating individual wire count and minimum specs.
Phone cable is typically Cat3, but Cat5, Cat5e, Cat6 and Cat6e can be used in a pinch, if you don't care about the extra cost. Each one is better (and more expensive) than the last. Cat5 is used all the time for phone wiring, as it can carry 4 paired signals instead of Cat3's 2 paired signals.
As such, a Cat6 patch panel can be used. It's just a way to patch cables after all, like a breadboard, but (hopefully) higher quality than a cat3 panel.
If you actually mean use RJ45 connectors, then you woul have to make custom cables with rj45 on one end and rj11 on the other, either male or female. RJ45 does not fit in a RJ11 hole, and RJ11 was not designed to properly mate with a RJ45 hole.