Is there a way to maintain/add Item Types?

Does the item class functionality provide what you need? You can use that for reporting and filtering and assigning defaults like GL accounts, warehouses, posting classes, price classes, and various other attributes. If not, what specifically is missing?

We use item classes for different classification purposes, and has many more layers than what we need.  I am hoping to use the Item Type, more or less specifically for non-stock items, to allow easy filtering of the parts for an API.  For example, we don’t want Expense items to be exposed.  Yes, we could use the Item class, however the item class has been extensively used with more than one layer.  I was hoping that the Item type, being a simple “tag” could be used for the filtering we need.  However, if I cannot add more types to the system, I can’t use it.  That’s why I’m looking for a maintenance option.

Have you looked at using Attributes to create additional fields that you can define (including standard dropdown lists?)

https://help-2022r2.acumatica.com/(W(3))/Help?ScreenId=ShowWiki&pageid=f2ea392e-5710-4e11-8148-902a861f773d
 

• Create the Attribute fields that you need on the Attributes form
• Then add the attributes you want to track on each item class that it applies to. 
• Some can be mandatory and some can be optional

I agree with meganfriesen37. Attributes would a good option for this. Alternatively you could create a customization to add to the list of Item Types. I don’t believe there is a no-code way to change the list.