I have added a SMA connector to the Sodaq One and have removed all the resistors. Because the signal now needs to be routed to the SMA connector I have a different route and placed a wire over the pads of the resistors.
Here is the SMA soldered to the top side.
Here is the bottom side with the wire to the SMA connector.
The connector is a multicomp of Farnell http://nl.farnell.com/multicomp/19-70-1-tgg/rf-coaxial-sma-straight-jack-50ohm/dp/1342652
This connector has 5 pins, but the two for the bottom side need to be cut off, because they hit some components.
I have it connected to the Dutch KPN network and after this upgrade I have seen it connect to one or two new gateways. But I did not notice a big RSSI or SNR improvement. On SF12 it connects to gateways of KPN at 11km away, while the Sodaq One is just in front of the window in an urban area.