If it is a constant length, then
SET @h = HEX('jkl');
SELECT MID(@h, 1, 2),
MID(@h, 3, 2),
MID(@h, 5, 2);
+---------------+---------------+---------------+
| MID(@h, 1, 2) | MID(@h, 3, 2) | MID(@h, 5, 2) |
+---------------+---------------+---------------+
| 6A | 6B | 6C |
+---------------+---------------+---------------+
SELECT LEFT(@h, 1),
LEFT(@h, 2),
LEFT(@h, 3),
LEFT(@h, 4),
LEFT(@h, 5),
LEFT(@h, 6);
+-------------+-------------+-------------+-------------+-------------+-------------+
| LEFT(@h, 1) | LEFT(@h, 2) | LEFT(@h, 3) | LEFT(@h, 4) | LEFT(@h, 5) | LEFT(@h, 6) |
+-------------+-------------+-------------+-------------+-------------+-------------+
| 6 | 6A | 6A6 | 6A6B | 6A6B6 | 6A6B6C |
+-------------+-------------+-------------+-------------+-------------+-------------+
If it is variable length, then consider a stored procedure, where you can loop.
Better yet, do it in Perl or PHP.