There are several ways to use a 50ohm network analyser to make measurements in a 75ohm system. Not surprisingly, they are all inferior in some way or other to using a 75ohm analyser, but they'll work within their limitations.
1) Measure everything in the 50 ohm system, and convert mathematically.
Pros - no extra hardware
Cons - you might want best accuracy around the matched condition, and this doesn't give that. Small errors in calibration will degrade your near-matched results proportionately more than large reflection results. The need for results conversion means an extra step offline, so you don't get meaningful final measurements in real time on the analyser.
2) Convert your 50ohm ports to 75ohm ports using 50/75ohm RF transformers.
Pros - power is not lost so noise level stays OK.
Cons - transformers will degrade match so reducing measurement accuracy, and only work over restricted frequencies anyway
3) Convert your analyser's ports to 75 ohms using min-loss 50/75ohm pads
Pros - much wider bandwidth and more accurate than transformers
Cons - they lose significant power, so your dynamic range drops, and S11 measurements are made through twice the pad loss, so you're losing sensitivity
4) Calibrate it with 75ohm calibration pieces
Pros - work at full power, mathematically turns it into a 75ohm analyser
Cons - you need a computer corrected network analyser (CCNA) and a set of 75 ohm cal pieces (expensive, depending on accuracy). As you're working a long way from the intended impedance, second order effects will mean it doesn't calibrate as accurately as would a dedicated 75 ohm analyser.