How to vertically and horizontally scale WebSphere Application Server on Bluemix?

I do not see any way to add resources to existing nodes, neither adding new nodes.

Adding new nodes may be possible by maintaining WebSphere Application Server node container image, but is there an easier option?

When creating a new WebSphere Application Server ND Cell you can click the "Deployment Manager" tab and scale vertically by selecting a T-Shirt size for your deployment manager. The size descriptions will show below as you click on the sizes. It is currently unavailable to scale an existing resource.

For the nodes you can click the "Application Nodes" tab to scale the nodes vertically (T-Shirt size) or horizontally (Number of application nodes)