First of all, you don't need an external antenna for this unit. From what you say, it sounds like you are already aware of this.
If you can add an external antenna to this unit, you'd have to open the unit up and there would be a port on the circuit board. If there is not a port, there may be a location to solder in a port.
If the receiver is dropping out, you might consider a different location in your vehicle. Make certain it has a clear view of the sky and is ideally not behind any metallic-based window tint.
I have a different GPS unit, and the receiver sits on my dash and I get rock-solid satellite reception.