Hello.
S2-L1C products are already with reflectances.
If you need to perform atmospheric corrections with Sen2Cor and Amazon S3 data, then yes, it is possible.
Please see this post in relation with how you can download S2-L1C products / data from Amazon S3 into SAFE format.
I have recently used such a downloaded product for input into Sen2Cor and the processing went OK.
Cheers,
Cosmin.