You can do this with padding. If cropping removes part of the photo you need to leave, you can instead get the same shape (that is, aspect ratio) by adding to the other edges.
You can use physical matting (creating a separate frame-with-the-frame) to put your original-aspect-ratio image into an 8×10" frame. Or, I have some nice borderless glass "frames" where the image appears to float in the center; with this style, it doesn't matter so much if the image isn't the same aspect ratio.
Or, you can alter the image digitally to add a border (possibly only on the sides that need to be extended). Depending on your needs, this border can be flat color (white, black, or a key color from the image), or you could possibly clone something (perhaps using "smart fill") from the image. This will work particularly well if the background and edges are simple, but will be difficult otherwise.
I think it's better to use matting (or as Michael Clark suggests, use a frame in the correct aspect ratio initially), because a digital border can look ugly. Or (also as Michael says) better yet to shoot the image in a way that allows the desired crop in the first place — but of course that's not always possible.