When you choose Convert Image/Archive… from the File menu, DropDMG brings up a window where you can select a source disk image or archive. Hold down the Command or Shift key to select multiple source items. DropDMG will convert each source item into a new disk image or archive, using the options from the selected configuration.
You can use Convert Image/Archive… to join a segmented disk image (a .dmg file and multiple .dmgpart files) into a single .dmg file. Select the desired format in the preferences, and make sure that Limit segments to is unchecked. Then tell DropDMG to convert the source .dmg file.
A shortcut for Convert Image/Archive… is to drag the source disk image or archive onto the DropDMG window or DropDMG’s Dock icon.