You can actually do it manually, as long as you put the "mimetype" thing as the very first thing in the zip archive (uncompressed) and add the rest of the files in after it (can compress).
See
this thread for an example of how the two commands to do this look like on a command-line interface, and any decent GUI zip creation app which lets you selectively add and delete files should be able to do it as well.
ETA: You'd have to create yourself an OPF and preferably NCX file as well for the manual approach, so you might prefer Sigil after all.