At first glance I think this might be possible (using a custom Claim Provider and implementing FillClaimsForEntity), however I'm not sure how or if it's possible to create a secondary identity claim, and fool SharePoint into thinking it's a genuine identity claim from whichever identity provider. Also - I don't know if SharePoint can even handle multiple identity claims in an identity.
Your next problem, if it is possible, is how you plan to map between FBA and Windows accounts, are you going to keep a mapping table somewhere? Your claim provider will need to look up this mapping table and create the appropriate second identity claim.
If SharePoint can't handle multiple identity claims, you could probably authorise users using a custom claim provider (ie: entries from your claim provider are used from the People Picker and granted access to the Site Groups), and your claim provider's FillResolve methods could perhaps resolve the appropriate claim (via the aforementioned) mapping table depending on the incoming claim.