It is correct that you can not choose signature after you started the reply using the "None" for Signatures and if you only have one signature on file.
It will Automatically use NONE if that is your original setting in signatures.
Here is my work around that:
First the solution will allow you to normally start replies without Signature.
If you now decide to Sign, you will have the option to choose a Signature.
To do that you just make a empty Signature in the Mail Preferences Signatures, using the + to add and name it something other than none (I called it test)
Now you should have 2 signatures to choose from.
Your original and the new one containing nothing.
In Signatures window choose the new one as default.
Now in replay to email, you will have no signature by default, but you can switch to the actual signature and it will be on the top of the Quoted message.