Bob:
I'm sure you've already discovered that the solution is your typical Copy and Paste [or Command C and Command V, if you prefer that manner of copying]. As I've described elsewhere, when you generate your shape in Pages [or *any other program that creates shapes] deselect everything, such as Border and Shadow, since those you can add back on in iWeb. But you do need to create the Fill in Pages before you transfer it to iWeb. Once in iWeb you can still change proportion, add a border and add shadows and duplicate endlessly. For some folks this may mean doing this just once for an entire website. [One could create an unpublished iWeb page with a number of pre-formatted shapes.]
If you use non-rectangular shapes [such as a circle], you will need to do a bit more experimenting if you wish to have a border. You may need to add that, too, in Pages, since iWeb treats the border as a rectangle. Luckily it treats shadows correctly.
*This trick works with any program that generates shapes, including MS Word and even EverWeb [for those who like a sense of irony]. I picked Pages because everybody is likely to have it installed on their computers and because its functionality is similar to iWeb.
But for the more serious graphic designer I have to share the good news: Canvas Draw [from ACD Systems] is back for the Mac [after a lapse of several years] and it is better than ever - I mean it is stunning!. For those who have been told they need Adobe Illustrator but gasp at the cost and complexity, Canvas Draw is the inexpensive answer. It combines precision technical features, such as an architect's CAD program, with freeform illustrating capabilities, opening up a whole new potential for beautiful website design. Anything you create can be imported back into iWeb. [No, I don't represent the company. Just like all of you, I simply want to create the best designs possible, where I'm in control of the program, rather than the program controlling me.]
Happy New Year!
Richard