I do this often.  Just have Pegasus Mail insert the signature in the reply (Tools -> Options -> Signatures, check "Add this variant upon message creation" near the bottom of the options page), then you can cut-and-paste the signature up where you want it.

Or you can use the glossary method.

Hello,

is it possible to insert the signature on the top if i write a reply mail?

for example Mailtext:

Hello,

...........

Signature

>old mailtext

>old mail 

>old mail

Pegasus put the signature on the bottom after the marked message. 

Hello!

Right now (in Pegasus Mail 4.41), this is not possible: a signature is entered at the very end of a message, no matter whether you have written your reply before or after the original text.
However, you can take advantage of the Glossary that Pegasus Mail has. The idea of Pegasus Mail's Glossary is that you define an abbrevation that can be expanded to a full text. For example: you would define "sig01" as the abbreviation, and that would be replaced by your full signature.

Go to "Tools" | "Glossary" | "Create/edit...". Press "Add" and enter something like "sig01", then press "OK". The abbreviation is being highlighted; just enter the text you want to have as your signature (note that you can enter simple text there, no HTML- or Rich Text, as far as I know). Do not forget to save the new Glossary entry.
When writing a message, type in the abbreviation (for example "sig01"), then press CTRL+E which replaces the abbreviation by the full text you have defined - you can do so in the very line you want to have your signature text to be added.

This may not be a signature in Pegasus Mail terms - but the recipient will not see the difference. If you want to follow this advice, do not forget to disable the actual Signatures - one way to disable them is simply to choose "No signature" when writing a message.

Just to say that I too would like to have a switch so that formatted signature can be added at start of reply as per original poster.
 
I currently use glossary and have several "signatures" defined there.  One issue I have is that I have been unable to apply text formatting (e.g. bold, italics) to any glossary text.

I have two clients that I work for and they insist that my signature is formatted as per their brand - no problem with Pegasus signature but impossible with glossary signature. I forgot to cut and paste signature on one email last week and have now had request to use Outlook for emails on their behalf.

Yes, I know it is unreasonable etc, but they do pay my wages (I freelance).  They also  are one of a number of people pressurising me to use a calendar - such as Outlook. I hope that v5 arrives soon and meets my needs otherwise my 10 years use of Pegasus will have to end.

 
Dave