This is something you might consider and explore.
Assume you have a SharePoint list with the Title column being a list of countries.
In a Power Apps app bind to that list
Added a button and create a collection ClearCollect(colCountries,Countries.Title)
Add a combo box, set the items property to the collection colCountries.
Add a button with this for the OnSelect Property
If(IsBlank(LookUp(colCountries,Title=ComboBox1.SearchText)),Patch(colCountries,Defaults(colCountries),{Title:ComboBox1.SearchText}))
If the SearchText returns no results, add the SearchText to the collection. Later the List can be updated to include the added values.