You can already do this with edge labels. Each edge can have an arbitrary number of edge labels, and with the placement properties, you can even specify that the multiplicity labels should stay near the start and end of an edge then you move the nodes. Here is a small sample graph: edge-with-multiplicity
Once you have created an edge with labels that fits your needs, you can use it as template for edge creation. To do this, create a new palette section, add the edge to that section with the right-click menu of the edge, and mark it as default for new edges by double-clicking its icon in the palette.
Please see the yEd manual for details:
