Yes, of course, it is possible to write text inside a node.
If the text is placed outside the node, you need to adjust label property "Placement". Either use "Internal: Center" or "SmartFree: Anywhere". The latter supports text inside and outside of nodes, so when your text is outside and you switch to "SmartFree: Anywhere", it will stay outside. However, you can then drag the text inside the node with the mouse.
You can change label placement for all selected nodes or labels at once.