Since velocity is a vector quantity whose magnitude is expressed as displacement as a function of time, you could do something for example, like assign a displacement value of 1 under your dynamic load case and apply a TH function to that load case equal to time vs. displacement to account for initial velocity. As you probably know, you can select and assign displacements to whichever areas and/or joints you wish on a load case by load case basis.
You can combine the results of that dynamic load case with whatever other load cases you want to combine it with.
If you are looking to first assign an initial displacement and then apply additional velocity loading you have a couple of options: 1. assign your initial displacement under one of your static load cases, then apply additional velocity as described above 2. or if initial displacement is a dynamic load case, create a load case and apply a TH function with time vs. displacement equal to initial displacement, then apply velocity
If you're doing a nonlinear analysis (you didn't say), you could start from a previous TH case to apply additional velocity load after initial displacement, or, I think you may be able to also use Define >Static pushover case, and define a load pattern with static and dynamic load cases..dynamic on a mode by mode basis (not completely sure how it works though), even if your nonlinearities have nothing to do with plastic hinge pushover analysis..Under analysis menu, you would run pushover. Probably ought to contact CSI for more info on use of nonlinear pushover options for non-pushover application
I heard CSI is coming out with a new version of SAP in a couple of months which will enable us to run any nonlinearity as a static or TH case. Right now, although it's accurate, it's a bit inconvenient to "fake" a nonlinear TH case to equal a nonlinear static load case by having to use .99 damping and flat line TH functions.
Sorry for such a long reply...this moonshine makes my head spin