In EnergyPlus you can use the AirloopHVAC equipment that model heat pumps (or ZoneHVAC if modeling a single zone). See:

AirLoopHVAC:UnitarySystem (can model any coil types) AirLoopHVAC:UnitaryHeatPump:AirToAir (only for HP's) AirLoopHVAC:UnitaryHeatPump:WaterToAir AirLoopHVAC:UnitaryHeatPump:AirToAir:MultiSpeed

Also see the \EnergyPlusVX-X-X\ExampleFiles folder for the ExampleFiles-ObjectsLink.html file to find which example files show use of these heat pump objects. And review the InputOutputReference documentation for each object (\Documentation folder).

I only see 3 methods for using ExternalInterface.

ExternalInterface,
    \memo This object activates the external interface of EnergyPlus. If the object
    \memo ExternalInterface is present, then all ExtnernalInterface:* objects will receive
    \memo their values from the BCVTB interface or from FMUs at each zone time step.
    \memo If this object is not present, then the values of these objects will be fixed at the
    \memo value declared in the "initial value" field of the corresponding object, and a
    \memo warning will be written to the EnergyPlus error file.
A1;  \field Name of External Interface
    \note Name of External Interface
    \required-field
    \type choice
    \key PtolemyServer
    \key FunctionalMockupUnitImport
    \key FunctionalMockupUnitExport
    \note Currently, the only valid entries are PtolemyServer, FunctionalMockupUnitImport, and FunctionalMockupUnitExport.

Thank you for your answer.

I have already tried these objects but the problem is that I want to implement a co-simulation scheme in which I should be able to control my Heat Pump from matlab through bcvtb and give "from outside" the values of fan speed and inlet damper position (or even compressor speed) for the heat pump. From what I have seen, these capabilities are not given in energyplus objects of HP: it is necessary to define a certain thermostat and you cannot control the fan speed and inlet damper opening .

kind regards