Is it possible to remove or better replace annoying title "extra field" with something more convenient, like "Parameters" or "The above set includes" followed by my extra fields?

Yes,
I dont know where exactly you want to change it, but it possible if you do some language overrides.

More on overrides can be found here. docs.joomla.org/J2.5:Language_Overrides_in_Joomla

To find the language string you wish to override open K2's laguage files.