Hi,
You can "copy" a tile without decompressing but you will always need to decompress before you can paste it back in and even if it were possible to paste a tile to a compressed project you would still need to decompress before you could use the tile editor to actually (pixel) edit it.
(nb. copied tiles are always buffered in memory in a decompressed state).
These restrictions are in place for extremely good reason, so if you want to do these things then you will have to change your methods to allow for the decompression to occur, ie. if you have "other characters" (non tile chars) in the set then try keeping them in a separate project and combining them at a later time to create the final set for your game level.