The IDictionaryAdapterFactory defines 2 overloads of GetAdapter that take a NameValueCollection.
In the interface the methods are behind the directive FEATURE_DICTIONARYADAPTER_XML IDictionaryAdapterFactory.cs#L64.
But the implementation shields only overloads that XmlNode DictionaryAdapterFactory.cs#L87.
Since System.Collections.Specialized is already package reference, i think the 2 overloads could be exposed. Otherwise a new FEATURE_DICTIONARYADAPTER_NAMEVALUECOLLECTION could be introduced.
I'm willing to contribute a PR.