Zip compression only. If your RAR/7z program also happens to be able to create and modify zip files, then you can use it, but ePubs are really just glorified zip archives with the internals arranged in a particular manner.
You'd actually probably have to tweak the existing OPF in another ePub enough for metadata like author, title, etc. that it'd just be easier to create your own OPF. See the
Wikipedia example for a version you can quickly copy-paste and substitute your own values into.
Making a simple ePub by hand from scratch with just a text editor and a zip program is actually very easy. Here's the website of someone who has
a very easy tutorial which shows you how to do it in plain simple language. You can skip the part which mentions ePub Check if you're just starting out and seeing how things work before committing to distributing or whatever you plan to do with your book.
ETA: Divide large books into chapters. Some older readers can handle reasonably big files, but only if the internal HTML is split into what it considers manageable "chunks".