Can you embed a html code into a signature? any apps that can do that?  

Adding your own HTML code is disabled in forum signatures and it provides you with the basic things you would use HTML for, what exactly did you want to add in yours?

When editing your signature you have an interface that allows you to mess around with the font and add things like images, everything it allows you to use is pretty much the only stuff you should be putting in a signature anyways.