Is there any way to add signature or footer text in category-specific topics?
No. There is a plugin.
This is for individual posts of a user. I would like to add footer for all topics posted in a specified category. Is there any way for that?
Not that I’m aware of. You can create a category-specific template, but to do what you’re describing you’d need a plugin or component.
category specific signature is possible?
You can create a theme-component to enable that. Or commission one in the #marketplace