In case you need to Freeze any particular row anywhere in the sheet you can use (Within org.apache.poi.ss.usermodel.Sheet) (Available in POI 3.7 as well)
Sheet.createFreezePane(int colSplit, int rowSplit, int leftmostColumn, int topRow)
In your case if you want to freeze just your first x rows then the int leftmostColumn, int topRow section will get removed and you can use just
Sheet.createFreezePane(int colSplit, int rowSplit)
for example
sheet1.createFreezePane(0, 5); // this will freeze first five rows