Dear Nam,
Thank you for your query.
On one hand, you can use the Windows API to automate the handling of any dialogue you want in any way you desire:
http://thebuildingcoder.typepad.com/blog/about-the-author.html#5.32
http://thebuildingcoder.typepad.com/blog/2009/10/dismiss-dialogue-using-windows-api.html
On the other hand, the AddIn Manager source code is not publicly accessible, and I am not aware of any possibility right now to modify its behaviour in any way at all.
There is an effort underway, or a request from developers, at least, to publish the AddIn Manager source code:
https://forums.autodesk.com/t5/revit-api-forum/why-add-in-manager-in-the-revit-sdk-2017-2017-1-has-v...
On the third hand, everything that it does you can also achieve yourself using standard .NET functionality.
If you are not completely satisfied with the AddIn Manager behaviour, you might want to consider implementing your own loader instead.
I hope this helps.
Best regards,
Jeremy