I have my signature prepared in HTML. Is it possible to add signature in HTML format in Pegasus Mail?

Yes. There are four signature variants, two of which are for HTML format (formatted content). The local variants apply to messages sent to users within a multi-user Pegasus Mail installations while the internet address variants apply to messages to external addresses. If you use a picture in your signature I suggest reading the Pegasus Mail manual topic about signatures because it contains a hint about formatting pictures in a signature that is not in the Pegasus Mail Help system.

Something to be aware of is that the Pegasus Mail option "Disable all text styling options..." will disable HTML formatted signatures.

One more thing you need to know: You cannot import HTML directly into the signature editor, you will need to convert it to RichText (RTF) format if you don't create it manually. Office applications like LibreOffice can do this for you, you don't even need to save it to file, just import your HTML into the Writer module and copy the result to clipboard, it will automatically provide RTF format with the embedded picture as one of the available options for insertion.