There are a number of ways to add files to an archive using Opus.
Using the context menu Add to Archive command, or selecting the Archive Files command from the toolbar, displays the Add to Archive dialog.
The easiest way to access this functionality is via the context menu. In the Archive Context Menu page in Preferences you can turn on or off context menu commands for adding files to various archive formats. These commands will be shown whenever you right-click on one or more files and folders.
These commands will add the selected files and folders immediately to a new archive. The new archive will be located in the same folder as the source files, and it will be given the same name as the first selected file.
For example, if you right-click a file called Presentation.doc the context menu might contain a command called Add To "Presentation.7z" - selecting this command would create a new 7zip archive called Presentation.7z and add the selected file to it.
The Preferences page lets you choose which formats an Add to Named Archive command will be displayed for.
These commands will also add the selected files and folders to a new archive, but instead of immediately creating the archive you will be given a choice of filename and path, archive type, and various parameters specific to the archive format chosen.
The Preferences page also lets you choose which formats to add these commands for although this selecting say Add To Archive (RAR) only sets RAR as the initially chosen format; the Add to Archive dialog lets you change the selected format.