To start with somewhat confused by your post, so hopefully this response will help.
I assume today you have a standard LAG/AE from the QFX5100 you want to replace, to something. I am also going to assume that something also is configured as standard LAG/AE/whatever, and not some form of MC-LAG type design/configuration. I believe best approach is to configure a 2nd LAG/AE/whatever on the "somethind device" and then connect EX4600 via it's own LAG/AE. Then you move cables from QFX5100 on to EX4600 - there will be some disruption but can be minimized if done properly.
Now if you question is if you can create a MC-LAG pair between QFX5100 and EX4600, and then attach devices to each either A/P or A/A and the cut over that way (which would be nice and basically non-disruptive) that answer is unfortunately NO. MC-LAG pairs are only supported on same device. Now "might" this work in non-supported fashion, do not know anyone who has done this, but easy to test and paly around with to see if it might work. You would want to minimize the time this MC-LAG [non-support] config was being used, of course.