If you mean copying or substitution, you can easily do copying. As for substitution, the capability is more limited.
To copy: outline select, cut (scissors icon, right click), switch to new schematic/board, paste. The copy to a board is complicated by the restrictions on backward compatibility. If it is a board without a linked schematic, it is no problem. Connections are not automatically made.
Also, the copy sequence is really useful in building new parts.
Substitution: You can create alternative packages and use the change command to change from one to the other on the schematic or board. Connections are maintained when changing packages or technology by tha method.
John
Edit: If you want to maintain connections, which you cannot do with the cut/copy sequence, you might consider making the item you want to instance part of a new device. So far as I know, the symbol and package programs do not have limits on size...maybe a single page. Unfortunately, you cannot import directly a schematic to the symbol. The nets do export, but you need to redraw (or copy/paste) the devices.