From VFXPedia

Inserting Tools

Use one of the following methods to insert a tool into an existing connection between tools.

Drag And Drop

Click on the tool to insert from the bins or toolbars and drag it into the flow. While still dragging the tool, position the mouse over the pipe of the tool to be connected so that the pipe is highlighted in blue, then let go. This will cause the new tool's first input to be connected to the upstream tool and the output to be connected to the downstream tool.

Shift Insert

If the tool to be inserted is already present on the flow, hold the Shift key down and drag the tool over the pipe where it should be inserted. When the mouse is positioned correctly, the pipe becomes highlighted. Release the mouse button to insert the tool.

From Toolbars And Menus

Select the tool in the flow to which the new tool will be connected. Click once on the toolbar button of the tool to be added, or select the desired tool from one of the tool menus. The new tool will be added immediately after the selected tool.