Force widget

If they’re all using the same theme, hard code it into there.

they are using different themes almost all of them.

is there any other way of adding content to the sidebar?

filters/hooks and others?

Not… really.

See, the header and footer, you can totally hook into with a mu-plugin (see https://wpmututorials.com/plugins/how-to-hook-into-the-footer/ for an idea). But Sidebars, as I’m sure you’ve noticed, are different per-theme. So you’re stuck editing the theme’s core files to put YOUR widget hard coded into each theme.

There’s no hooks for sidebars – not for multisite or single site. Like Ipstenu said – this is front side and every theme is different.