What you want to do is no problem. As maps4gps said, just keep sending the maps and renaming to files with names you will remember, like UStopo.img, CN.img, whatever.img.
However you need to be aware of the segment limit on your device, which will only recognize a total of ~4,000 of the individual tiles that make up the maps. This limit applies to the sum of all the different maps on the unit and not each separate file.
The DVD version of US Topo has about 6,600 segments in order to be compatible with older units that have very small amounts of memory. Therefore it will not be possible to install all of that map on your Oregon. When you use Mapsource and select an area of the map to transfer, you can see how many segments are being sent.
The version of US Topo pre-installed on the 550t has been created with much larger tiles and allows the whole US to available while using less than 500 segments. The downloadable and pre-loaded card versions are also similar.
City Navigator also uses pretty large tiles and I think it only eats up a few hundred segments. Segment size is chosen by the map author, so it's impossible to generalize about how many will be contained in any of the maps posted here.