Can I add X (Twitter) to Spanbox?

Yes, X (Twitter) is one of the apps Spanbox supports. Add it from the app list, sign in on X (Twitter)'s own page (WhatsApp is a QR scan), and it lands in your unified inbox next to everything else, DMs and your timeline. On the free plan it just uses one of your 3 app slots.

You can even run more than one X (Twitter) account at once, each in its own isolated session, so a work and a personal X (Twitter) sit side by side without logging in and out.