I am using javax.microedition.lcdui.Canvas for drawing my string on the screen. But I also need one dialog window for some purpose. So I am using lwuit
I would say it's possible but it will increase size of the app significantly. Whenever you need your dialog you can init LWUIT Display and use LWUIT Forms and Dialogs.
I would better to implement some really simple Dialog ourselves. It's not really much work. Or use another third party solution.