Pages 10.0, how can I add a signature?

If you do not have any signature annotations already in Preview, follow Apple's guide, Fill out and sign PDF forms in Preview on Mac.

In Pages 10 with a blank template, insert a rectangular shape. In the Arrange panel, set its provisional size to 2.75 in x 0.75 in, and back in the Style panel, set its Fill to No Fill, so you will have a transparent background. I recommend that you place a 0.5 pt black line on the shape. With it selected, copy to the clipboard.

Launch the Preview application and from the File menu : New from Clipboard. This will insert your shape from Pages, and with a transparent background, the need for those thin border lines is now apparent. I have set my tranparent background color in Preview to be the blue that you see below:

In Preview, show the Markup Toolbar, either from the Preview toolbar itself, or by pressing ⇧⌘A. On that Markup toolbar, will be a an icon with a signature on it, and as you pass over this icon with the mouse, the word sign will appear. Click this icon, and the drop down menu will offer the available signatures as shown below:

This will probably insert a very tiny selection box with your signature in it, and just grab one of the corner grips and expand to increase your signature size until it just fits inside the transparent shape borders. You can also change the signature color from the default black to any other color via the left of the two Preview Markup toolbar color adjusters.

Click outside of the signature box to set the signature onto the image. Now, on the far left of the Markup toolbar, is the Rectangular Selection tool. Once you have selected it, click once inside the shape border, and drag down and right until you have a selection inside of your shape borders. Click the Crop button on the Toolbar, and you now have a borderless signature. Select the File menu : Export… to PNG with √ Alpha and place the image onto your Desktop.

Remember, in my example images, everything that is blue is actually transparent.

You can now drag and drop this signature PNG image onto your Pages document. From the Pages Arrange panel, set Text Wrap to None, then size and position as needed for your document.

Good. You solved it.

Ordinarily, I am not one for labyrinthine goal achievement steps, and the process that I provided was successfully tested with Pages 10 and Preview on macOS Mojave 10.14.6. Your approach by exporting to PDF, and then providing a signature annotation from Preview is the simplest activity where you do not require a signature as content to the original Pages document.

In Preview, close the open PDF from the File menu, and then quit Preview from its application (Preview) menu. Now, In Pages, select the shape that now has No Fill, and a thin border, and copy to the clipboard. Relaunch Preview, and the New from Clipboard menu item will be available. Choose it to open the Pages shape in Preview.

This would all be far simpler if you had access to a digitizer tablet that generated an image of your signature, but I couldn't assume that was available.