You can use a fan leak to add a fixed momentum to the gas in the defined location without neither adding or removing any mass from the domain.
A practical situation where you might encounter this, is an air exchanging fan between two rooms.
A fan leak in FLACS should be aligned with one of the axis directions. If the leak direction of the fan is normal to a solid plate of zero thickness, the flacs simulator will open the control volume corresponding to the position of the fan to allow gases to flow through.
The articles What are leaks, How to define a point leak and How to define an area leak have general information about leaks.

Define a fan in CASD

A fan is defined as a leak in CASD. You can either define a leak with the Leak Wizard or by adding a leak in the Leak section in the scenario side bar of CASD. Set the leak type to fan, and define the other parameters as for other leak types.

Specify the position, leak box size (set the size to "0 0 0" in order to define a point source/sink), and direction.

The Leak Wizard in FLACS

The leak outlet parameters are similar to those for other leak types. You must give the leak area, the mass flow rate or the velocity of the leak, relative turbulence intensity, turbulence length scale and direction cosines. If both the mass flow rate and the velocity parameters are specified then the mass flow rate will take precedence, since only one of the two is needed. Suggestions for the relative turbulence intensity and turbulence length scale are 0.1 and 10% of the fan diameter, respectively. An ambient temperature can be defined, but the temperature of the gas blown downstream of the fan is inherited from the upstream gas and will not be influenced by the defined temperature. The start time and duration of the fan leak also have to be defined.

Outlet parameters window for a fan leak in FLACS

If the flow rate of a leak is specified as velocity (rather than mass flow) for a FAN or SUCTION leak, then the mass flow rate is calculated from the molecular weight of fuel rather than the one of air.