How can I insert (not replace) a byte at zero index in ByteArray ?

Assuming the existing array is already full: Make a new ByteArray with length equal to your current ByteArray + 1. Use

System.arrayCopy()

to copy the existing ByteArray into the new ByteArray starting at index 1. Set index 0 equal to your new value. Note that if performance is a concern you probably want to figure out a way to avoid doing this operation.

Instead of

System.arraycopy

(which will work) you should use

oldArray.copyInto

which is available in the kotlin stdlib on all platforms.