If you want them all as a single sprite, and assuming your sprite is using a repeated grid size you can create an empty sprite and then right click and choose "Edit Animations", then in the "Animation Frames" window, right click and choose "Import Sprite Strip". It will ask you for the number of vertical and horizontal cells and then import them in as individual frames. You then drag the sprite into the layout and set its "initial frame" to match the cell you want in your animation.
That last step is a bit of a pain, but I don't know of a way around it as C2 doesn't have a tile editor.