First how are you planning to connect the external GPS to your iOS device?
Because you either have to purchase an external GPS that already has the capability to connect to an iOS device, or the GPS device can transmit the location via Bluetooth Low Energy (LE).
Apple doesn't allow any device to communicate with their iOS devices unless it's part of the MFi program.
After that placing the coordinates on to the map is a piece of cake, first you have to keep in mind that any GPS device will give you coordinates in Lat/Lon (Spatial Refrence with WKID 4326), so you'll have to project that to your map's coordinate system.
After that you'll just create an AGSGraphic and place it on the map with the geometry and symbology that you can define.
Here is some code to do that :
AGSPictureMarkerSymbol *gpsSymbol = [[AGSPictureMarkerSymbol alloc] initWithImageNamed:@"ArcGIS.bundle/GpsDisplay.png"];
AGSGraphic *gpsGraphic = [[AGSGraphic alloc] initWithGeometry:<Your Geometry> symbol:gpsSymbol attributes:nil infoTemplateDelegate:nil];
Then just add the gpsGraphic to the map.