There’s nothing that really works like extendSelection, but you can use SelectionRange.extend to extend a single range, and you do view.dispatch({selection}) to set a new selection.
You can read view.state.selection.ranges or view.state.selection.main to get the selected ranges (or the main range), and do view.state.sliceDoc(range.from, range.to) to get the part of the document covered by a given range.