Which version of FEAP are you using?
Most versions support use of proportional loads.
1. Give value of a 'base' loading during mesh input. Use: FORCe, EFORce, CFORce or DISP, etc. options or CSURf for distributed loading
2. Assign loads to individual proportional loads FPROp, CPROp type commands to assign each degree of freedom.
If only one load multiplier is used this is not required.
3. In command language define the time variation using PROPortional load commands. Each dof gets multiplied by its proportional load value: F(t) = p(t)*F where F is the base value and p(t) the proportional loading.
Other commands exist. See user manual for more details.
