Seekbars in Android cannot have negative values. Assuming you have specified a value of '100' for your maximum range of the progress bar (for example using setMax(100) or in your xml file using android:max="100"), then you can use this method to move your progress bar to the middle whenever you want (for example in your Activity.onCreate() or wherever else)
yourProgressBar.setProgress(50);
After that you can get the progress value using:
int progress = yourProgressBar.getProgress();
If you want to have a progress value between (e.g.) -50 to +50, then you can simply use:
int progress = yourProgressBar.getProgress();
progress -= 50;
Good luck.