Maybe is it not what you exactly looking for, but it you what to copy border style from one table to another you can use this method:
private void copyTableBorderStyle(XWPFTable table, XWPFTable newTable) {
newTable.setInsideHBorder(table.getInsideHBorderType(), table.getInsideHBorderSize(), table.getInsideHBorderSpace(), table.getInsideHBorderColor());
newTable.setInsideVBorder(table.getInsideVBorderType(), table.getInsideVBorderSize(), table.getInsideVBorderSpace(), table.getInsideVBorderColor());
newTable.getCTTbl().setTblPr(table.getCTTbl().getTblPr());
newTable.getCTTbl().setTblGrid(table.getCTTbl().getTblGrid());
}
But for your question if you what to change outer border you need to get org.openxmlformats.schemas.wordprocessingml.x2006.main.CTTbl property and configure it:
CTTbl cttbl = table.getCTTbl();