It’s a little kludgey, but it can be done.

If you have a Things project with a bunch of items under it, first create a project in Omnifocus that will be the destination project.

Go to Things and open up the project. Select all of the items and then drag them to the Omnifocus icon.

You’ll get an Omnifocus import dialog. In the dialog you’ll notice that all of the items have been split out on rows in the notes field. Select and copy this text and then cancel the import dialog. Alternatively you can just drag the projects to a text editor and copy the lines from there.

Go into Omnifocus and paste the copied text into the main area. Omnifocus will paste the items as projects, but they’ll all be selected. While they’re all selected, grab them and drag them on top of your container project. When they fall into the container context they are converted into tasks.

Due dates and notes and things like that are naturally lost when you do this, but it does beat typing everything over again.