You can loop a footage item with the Interpret Footage dialogue in the Project Panel.
If it’s stock footage, though, it probably won’t loop seamlessly. You can try setting your in point for the layer in the middle somewhere, duplicate the layer and position the duplicate down in the timeline, dragging out the layer handle back to the original in point, then cross dissolve the end of the top layer to the beginning of the lower one. Then set the end point of the bottom layer to be one frame before the in point of the top layer.
Now render this out, re-import it, and set it to loop as many times as you want.
Joey Foreman
Editor/Compositor/VFX Artist