With a few tries right now I could see some used *cards*, but not some used expansions. However, I do not use this interface at all normally, so I guess it will come up from time to time.
I don't think what you literally asked for is possible, at the moment, but I encourage others to correct me who use this method to join tables.
However, I have the feeling that you don't want to know what expansions might come up out of the blue, I think you either want to seek out or avoid certain expansions, both of which is possible (to a certain degree) in the "Matching" tab.
If you want to avoid certain expansions, set them as unfamiliar in the "Familiar Cards" tab, then go to the "Matching" tab, select "Advanced options", then make sure to deselect "Rated game".
If you want to seek out expansions (instead of just base game cards), again at the Advanced version of the "Matching" tab, check the box "Wait for Expansions". That will make sure that your opponent (assumimg you only have the base cards) will have at least half the expansions. Of course, this is not perfect, because your opponent is free to preselect some or all 10 Kingdom cards.
If you have questions about anything I wrote, feel free to ask. If you can clarify why you want to know about expansions, it may be possible to achive that goal in another way (as I tried to explain above).