The program has a modular construction, with most of the package being independent of the particular problem being solved. The user needs to write a module which returns the appropriate element matrices for the problem in hand. Then the macro language introduced by Taylor is used to set up the solution algorithm.
See also Sanjay Govindjee's website which contains examples of user-defined macros, mesh commands, elements etc.
There is also the FeapUI project, which is creating a graphical interface for FEAP.