Apple - Is it possible to convert window into tab?

You need to drag a tab - Safari won't let you drag an entire window into another one. You can hit ⌘T to create a new empty tab (thus making the tab interface visible), then drag your original tab to the other window.

Go to Window → Merge All Windows. In Safari, this merges other windows into the currently active window. In Finder, this merges the current windows into other window.

Alternatively, you can show the tab bar with View → Show Tab Bar. This lets you drag the tab when only one tab in a window is open.

You can just hit cmd + shift + T to make the tab bar visible in any finder window. This allows you to drag the current window as a tab into any other finder window.