For adding expense after, this is where FAM really falls down. Once the asset is in service, there's not a great way to do it.
You can just manually update the asset value, but that doesn't generate any entry back to the GL. If you put in the GL entry (or vendor bill or whatever) first and hit the FA account, you can't append that value to an existing asset, the system would try to make you create a new one.
I think most people do just update the value though, even though it's really easy for your GL and FAM to get out of sync that way.