EPUB files can be created from any specially crafted GitWED websites. Essentially, you create a index.html file with the cover image and links to a bunch of chapters (or articles). Then you use Generate EPUB button from the orange ... menu.
Usually, you start by cloning an existing magazine issue (or rather empty version of it).
If you already have a type-set magazine, you can often export EPUB file from your type-setting program (InDesign, Quark Xpress etc). Make sure you set image resolution to the highest possible setting (usually 300dpi). When you'll be pasting images, they will be rescaled appropriately.
Next, rename say magazine.epub to magazine.zip and unzip it. You should get one or more HTML files. Open the HTML file in your browser (I have found Chrome to work well). Then, copy-paste articles into separate subpages.
When copy-pasting styles like italic and bold are preserved (this should work regardless if you used specially named character styles in your type-setting software).