MindMup allows you to add custom connections between unrelated nodes, with the help of ‘Connect to another node’ tool from the toolbar. Just tap the node that you would like to use as the start of the connection, then tap the ‘Connect to other node’ tool in the toolbar, and then select the destination node.
A quicker alternative is to select the start of the link, then hold Alt and click on the destination. Alt+click again will remove the link.
You can click on the link to change the colour, line type or add an arrow pointer.