Yes.

Editing a landmass is done using the same drawing tool you used to create it. Start the drawing tool, and if you look at the command line, it should offer you the option of pressing E for 'Edit'. Doing this will allow you to add to or remove from an existing landmass, by basically drawing the coastline again between two nodes. Just follow the prompts on the command line.

To merge two landmasses, you should delete the black outline of both landmasses first, then use Break [BREAK] to remove the parts of the landmasses where they will join (This will remove the fill of the landmass, don't worry about it, it will be restored), then use combine paths [CMB] to combine the outline paths into one path, then use Path to Poly [CLOSEPATH] to close up the polygon again (This will return the fill to the landmass). Finally, use outline in Black [OUTLINEB] to give the landmass an outline again. Now, if this results in some long straight connecting lines between the landmasses because they were a bit apart, use the Edit functionality as mentioned above to replace the straight line with a better coastline drawn by the drawing tool.