Are resistors the only legal way to tie various power designations together?
In schematic editors, a 'net', that is a set of nodes connected together, has a name, and is identified by that name. Therefore the 'VCC' and 'VDD' nets are distinct. If a wire is used to connect them, this will cause a logical problem for the editor. Different editors have different behaviour in these circumstances. Some will automatically rename one net, some will leave the nets unconnected.

There are several solutions

a) Connect them by a component, such as a 0-ohm resistor. This is wasteful if the only reason is to get round net naming issues, though the odd 0-ohm link in power supplies can be handy for debugging, allowing various bits of the circuit to be isolated

b) Choose a unified net name as one of the comments suggests. Rename everything to 'VCC' or the like. I don't like this as labels like 'VCC' and 'VDD' already have semantic meaning, and can cause confusion with the data sheet, at least, they confuse me.

c) Name the nets on the board for what they are. I tend to filter supplies, so end up with nets called '+5v_in', '+4.8v_fil', '+4.5v_fil2' etc, so I know where I am.