I have a question:
When I am doing a transaction and I send my signed tx to the network. Could someone else take that transaction and add a additional signed input to the tx. So this would mean that my tx gets coinjoined without my approval, would that be accepted by the network?
Greets
Not possible unless you explicitly chose to use non-standard sighash flags (and even then! possibly it could happen with SIGHASH_SINGLE|SIGHASH_ANYONECANPAY).
Or in layman's terms, by default every signature is signing the complete set of inputs and outputs. If you change any input or output the signatures become invalid.