Yes, you can.
Also note that a few of the PrE 9 Templates (Menu Sets) had issues, if one tried to modify them. It is my understanding that PrE 10 fixed those problem Menu Sets.
I would definitely not overwrite the PrE 10 Menu Sets with the same ones from PrE 9, but instead open the folder structure for the Templates for each version, and only add (keep the folder structure, but if that folder name exists, then maybe increment it, so as to not overwrite) the desired PrE 9 Menus. Note: there are several PSD files (the Menu Sets in a couple of formats), plus the PNG thumbnails, and perhaps Video & Audio Assets, if the Menu Set had Motion Backgrounds. Keep those in the same folder.
tell me please - PE 9 have more menu templates than PE 10? (222 MB more - hard compression? )
sorry for my english
I do not have a list of the Menu Sets, between those two versions, but others will likely have both versions, and can compare the Library items.
Historically, Adobe has kept the bulk of the Library Menu Sets fairly constant - the majority are retained from version to version. They might add a few, and remove a few, but the majority will be the same. As they are grouped in sub-folders in the Library, it should be easy to do a side-by-side comparison between versions.
I would NOT overwrite any of the Menu Sets in PrE 10, with ones from PrE 9, as PrE 9 had some issues. No sense in reintroducing the problems into PrE 10. I would ONLY Copy the folders (with files) of Menu Sets, that do not exist in PrE 10, and just place them in the Library folder hierarchy. Retain ALL files, including the PNG thumbnail files, so that you can see the Menu Sets in the "viewer" in the Create Menus Panel.