The Yahoo BOSS plugin makes it easy to create a search engine in your WordPress install. This plugin does not replace the default search.
You can add as many as you would like, but the Yahoo BOSS API will only support a limited amount. Each site url is added to the URL when performing the search, so there is a definite limit to the API. More sites can be added by using YQL.
This is not supported by the API. So this support would need to be built using another service.