Can TRACON increase plane speed on approach?
Yes, we can ask pilots to speed up on approach, but it is discouraged and not commonly done. This is because, as an aircraft approaches an airport, the crew will configure it for landing. This includes lowering the flaps, which lowers a number of reference speeds - essentially, making them unable to fly very fast. In order to speed up, they would have to raise the flaps again, which would result in extra workload and bad fuel economy.

The controller you heard knows this. They used the phrase "if able" to emphasize to the pilots, that they only had to comply if it was convenient for them.

ICAO provides the following guidance (form Doc 4444):


  4.6.1.3    Speed adjustments should be limited to those necessary to establish and/or maintain a desired separation minimum or spacing. Instructions involving frequent changes of speed, including alternate speed increases and decreases, should be avoided.
  
  ...
  
  4.6.3.5    Arriving aircraft should be permitted to operate in a clean configuration for as long as possible. Below 4 550 m (FL 150), speed reductions for turbojet aircraft to not less than 410 km/h (220 kt) IAS, which will normally be very close to the minimum speed of turbojet aircraft in a clean configuration, may be used. 
  
  4.6.3.6    Only minor speed adjustments not exceeding plus/minus 40 km/h (20 kt) IAS should be used for aircraft on intermediate and final approach. 
  
  4.6.3.7    Speed control should not be applied to aircraft after passing a point 7 km (4 NM) from the threshold on final approach.