RX and TX antenna
In most of the cases one of these is used for tx and both are used for rx, but one at a time. This is diversity reception. If you look at the PC card near the antennas you may see holes for a coaxial socket. If you do this is where to run the tansmission line for your new antenna from. There should be some pads at the end of the trace where a capacitor which goes to one of the other antennas can be moved to go to the coaxial socket tracce.
If there is no such set of holes you can cut the trace to each existing antenna and solder coaxial cable to them and run this to your new antennas.
How much attenuation do you think i will face if I used a RPSMA connector with cable as compared to direct sodering on the pads.
Do you think it is a good idea to desolder teh internal antenna cable right off the PCB and solder a 50ohm TV cable there and take it right upto to the dish antenna.
The attenuation of a socket will not be more than 1 dB. The attenuation of your coaxial cable will be several tens of dB by the time you get to the roof. You should consider moving your card to the roof and run the digital signals down on a cable.