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?
There's no real reason you can't. Just wire all the ports to each other, and you'll be fine. You don't want to swap connectors or anything, just wire everything in parallel.
This is exactly what a commercial phone splitter does internally.