The right-click, Install menu item for fonts (which is part of Windows) will appear for multiple files, as long as they are the same type.
e.g. If you have a mix of e.g. .TTF and .FON files, select & install all the .TTF ones, then do the .FON ones separately.
The Type column is displayed by default, so clicking its header to sort by it is probably your quickest way to group the files.
If the Type column is hidden, or if it shows the same thing for two different extensions, you can quickly sort the file display by extension by holding Shift and then clicking the Name column header. The arrow will point sideways to indicate this special mode.
Abr's suggestion of adding a button which runs Copy INSTALLFONT should also work, and might be preferable if you often need to install lots of fonts with a mixture of extensions.
A third way: I think you can also drag files to an Explorer window which is showing the Fonts folder. (Also works with an Opus window, but you need to make sure Opus is displaying Fonts as a virtual folder, not the real/phyical directory, while Explorer can only display it as one. That's configured under Preferences / Folders / Virtual Folders, by selecting either Treat all "virtual filesystem folders" as virtual, or by selecting Treat specific virtual folders as real... and making sure Fonts is not in the list.)