Is the font in italic in the epub?
If so the easiest way is to use the tweak epub function.
Hit T for tweak.
Explode epub.
edit stylesheet in notepad or whatever.
change all occurrences of italic to normal and save
preview epub to see if it is okay
and save
takes 2 minutes
Then make sure you don't have an original_epub file as calibre will use that(remove if there is one) and convert to lrf.
Converting to lrf will easily let you embed fonts BTW.
Helen