I actually found the solution for it. After some headbanging.
myproject.Application.CustomFieldRename(PjCustomField.pjCustomTaskText12, "columnName");
myproject.Application.CustomFieldPropertiesEx(PjCustomField.pjCustomTaskText12, PjCustomFieldAttribute.pjFieldAttributeValueList);
myproject.Application.CustomFieldValueListAdd(PjCustomField.pjCustomTaskText12, "ListValue");