No, there is no tool that allows such conversion.
I think you should consider several important factors.
First, most apps need at least some changes when adjusted to smartphone screens compared to desktop versions. While you can re-use a lot of code, if your app is architected properly, many if not all views may need to change. In some cases you may realize that required changes are significant (e.g. a tree on the left side with a DataGrid on the right side won't work on a smartphone screen).
Second, MGWT uses a different layout model. There is no equivalent to LayoutPanel, VerticalPanel, DockLayoutPanel, etc. - all of these layouts are achieved through flex-box layout model, which is fundamentally different.
NB: If you think that you app already looks and works correctly on small screens, there is probably no need to convert it.