Cara Nash Snyder sent this AutoCAD tip for creating multiple hatch boundaries.
If you select one hatch and right-click, you get the option to Generate Boundary, but if you select multiple hatches, you do not — unless you just did one, in which case Repeat Generate Boundary will appear in your right-click menu. So generate one, then you can select the rest and repeat.
Or you can just select all of the hatches and type in the command Hatchgenerateboundary (using AutoComplete, of course).
Notes from Cadalyst tip reviewer R.K. McSwain: This is a helpful tip because of the different menu options that Cara points out, depending on the previous command. Although the type-in command is long, I would put it in a ribbon or toolbar button, and then I wouldn't have to worry about doing one and then doing the rest.