Yes! – Since TRANSYT 13 you have been able to do this. The latest version of TRANSYT has an integrated Network Diagram that includes simple-to-use tools to build networks from scratch, or edit existing ones.
Using the mouse, network artefacts (traffic streams, controller streams, etc.) can be created in any order and then easily linked together – All within the interactive diagram.
A useful tool allows you to rename all items using specific naming conventions, so for example the names of Traffic Streams will reference, say, the controller that controls them, and/or the direction of travel using the eight points of the compass.