Hi Laura,
From what i have understood from the requirement you are looking for a field which can hold some custom field value in the SAP system .
You can have a look at the Train station field in the KNA1 table . The field is of the type CHAR , so it can take in any type of value.
Let me know if i have misunderstood the requirement.
Regards,
Vag Vignesh Shneoy