Is it possible that I select the Actors from the drop-down (U1,U2,U3) for a particular Task and assigned it.
Sure it this is possible. The jBPM api can be used to assign any actor any way you want it to.
For example, if I have three users U1,U2 and U3 (all are Admins) and three Task nodes N1,N2 and N3, then how can I assign Task N1 to U1, N2 to U2 and N3 to U3.
If you want to do this automatically, you can have assignmenthandlers in your processdefinition. In this assignmenthandler you can do anything you want as long as it is possible in java