If you would like to fill the mini keg with a finished beer from a larger keg:
First, chill the mini keg to the same or colder temperature than the beer temperature. This ensures that the beer will retain more of the CO2 as it gets lost to heat and evaporation.
Next, raise the CO2 pressure of the beer being transferred. If the beer is in a five gallon keg, raise the pressure by 10 psi and shake the keg three to four times for 24 hours to introduce a higher CO2 volume. By over carbonating the beer before it goes into the mini keg you compensate for the loss during the filling.
After the beer has acquired a higher volume of CO2, lower the pressure to 2 psi. Transfer the beer into the mini keg by filling the pre-chilled keg from the bottom up. Insert the bung and refrigerate.