Does this work for you?
AddPrefixToAllLayerNames.py (1.3 KB)
Right now there is no separator programmed between the typed-in prefix and the layer name. If you wish to have a separator like a dash or underscore, open the script in a text editor and change the following line:
sep=""
to (for example a dash)
sep="-"
i.e. just put the desired separator between the quote marks. Then resave the file and close it.