For developers, yes, this is a simple process. You simply increase the priority of the thread in which a task is running.
For the end user to increase or decrease a task's priority, the developer of the program would need to provide a slider bar (or other interface element) to access the program's thread priority setting, and then write code around that to increase or decrease the priority accordingly.
Note that setting the priority of a thread is not a "speed throttle;" setting a thread's priority very high will not make it execute faster, unless there is contention for the CPU, i.e., other things going on, too. And in that case, a high priority for a task will make other things (such as window moves, menu responses, screen redraw for other applications, etc.) happen more slowly, which may not be desirable.
For more information on threading, there's a Be Newsletter article at: