Hi Anthony,
If you click into the field and do ALT + F1, what does it say the field is called? It's not in standard Maximo asset app, but could be in one of the add ons.
Generally the values that you can choose from there are controlled by a domain or a class.
To find out which domain, you'd go to Database Configuration, choose the object name (from the ALT+F1 before the .) and then click the attributes tab, find the attribute name (after the .), expand the details and it will tell you the domain. You can then go to the domains application and you should be able to edit it and add more values that way.
It might not have a domain in, it might genuinely be tied to GL Accounts, in which case you'd have to go to the Chart of Accounts app and add a new component / account.