You can add a pre-existing flow to the flow you are working on, by simply dragging that flow either form the Bins or from an Explorer window into your flow area. It will then automatically add a Loader tool bringing in this flow. However, the pre-existing flow will need to have a Saver in it, and the results will need to be pre-rendered for this to work. Essentially the Loader that is bringing in that pre-existing flow is looking for a path in the Saver of where to find the rendered results.
Another option would be to box select a group of tools in one flow, right-click and from the context menu that appears choose Settings > Save all as and then choose a path and file name. You can then add those saved settings to a Bin and bring it into a new flow. This will add all of those tools once again to your new flow.
Please keep in mind that you can also box select and Group a collection of tools into a single tool to make organizing a large flow a bit more clean. This single tool can be expanded at any time to give access to the grouped down tools when needed.
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