We strongly advise against such a customisation. This is because of the extensive coding and functionality behind this field, how it interacts with Urgency and Priority fields, how it will influence SLO and Events functionality as well. Modifying this field to act in any way other than the intended functionality may cause unexpected and unpredictable behaviours within the product. Further, one cannot add or inactivate any entries in the Severity table either, without causing unpredictable effects to the product.
As for using one of the existing StringX fields that are defined in the Call_Req table, these fields are of a String data type. If the customisation only requires such a field defintion, then the StringX field can be used in this fashion. Other functionality, such as having the field derive its data content from another table, a SREL relationship, this is not a supported method of use as a field's definition on data type cannot be changed once it has been established.