Can a small voltage represent a closed circuit?
This is how the garage door opener probably works assuming a simple 2 wire switch with no extra features (I am not liable if you blow something/yourself up):







simulate this circuit &ndash; Schematic created using CircuitLab

Just a simple input.

Could you add a random 5V? No. One, without a common ground it won't really work. Two, Directly connecting 5V to ground would be a dead short and Bad News™.

Simplest Solution, wire another switch (or relay powered by a different source) in parallel. Done.







simulate this circuit