Yes, but it isn't easy. By that I mean, the worn out red blood cells being carried by the lymph system will have some iron in them. Therefore, you could magnetize them without manipulating them on a minute level. However, more aggressive manipulation of the white blood cells can be done; iron-heavy proteins can be inserted into the body, which can be taken up by the white blood cells. This allows scientists to track them via MRI, and would make magnetizing them possible. So, you can magnetize some of the components of the lymph.