If you only resize the sheet on the Y axis, this can easily be done with Blender's existing tools. In your image editor, resize the image upwards by 2x. For example if your sheet is 256x256, resize it to 256x512. In Blender load the resized sprite sheet and go to the UV editor.
In the UV editor, set the 2d cursor position in the 'n' key sidebar, by going to the view tab in the sidebar. Set the position to 0,0 and select all of your mesh faces. Set the pivot to the 2d cursor, and use the UV scale tool. Press Y to restrict the scale to the Y axis, and type in 0.5. This will restore your UV map to the original setup with the larger sheet.