Yes, you can use the Browse Teams and Channels when attaching a file into a conversation / reply. Click the little return arrow in the top left, till you get up to the Team level then you can go in and select files form that Team.
Obviously you need to take into consideration of permissions at this point. If you link a file to another Team, if the Team is not public, you want to make sure everyone that would be clicking your link to the file has access to said file or they will get an error about it and be able to request access to the other Team.
If permissions are an issue the best way to handle currently is to pop out on your files to "Open in SharePoint" and get a link to share, then go back into Teams and post a sharing link that you generate. This will be built into the UI once the SharePoint integration lands.
If you want the entire library from ex team B in Team A’s files folder, you can just add cloud storage and choose sharepoint - then add the library from Team B! But as @Chris Webb mentioned, you have to think about permissions