Contribution of solder parasitic to S parameter measurement
The solder itself won't bother much. 

The real problem lies in the fact you have to cut the cable and separate the conductors from the coax.  This will drastically change the line impedance at that spot.

If you have a VNA, you would be able to "see" such soldered junctions.  Not because of the solder, but because you had to separate the conductors and the insulators in the coax to be able to solder the connection.