Hello Nino,
I'm looking for an answer to this question as well. So far, I've found that you can manually override this read-only setting by following these steps when replying to a ticket. (At least with the way that my company set up SmarterTrack)
1. Click the More... button in the editing toolbar (Below the Subject line)
2. Click the Source button
3. Find the line that contains 'contenteditable="false"' and change this to 'contenteditable="true"'. (With my company's setup, this is located in this line: <div contenteditable="false" id="divSignature">)
This, unfortunately, has to be done for every ticket. I've yet to find a way to either set 'contenteditable="true"' or remove this tag entirely from employee signatures.
Hope this helps you out!