Strictly speaking, Traceur is not "removing" comments. Traceur parses the input es6, transforms the resulting symbolic tree, and writes es5 source from the result. The tree does not contain the comments and thus they do not appear in the output.
Attaching the comments to the tree is not so difficult in theory, but figuring out where to put them in the output is very difficult. Of course there are simple cases where, say comments precede a function declaration, but in general the input may not be closely related to the output and where do we put comments in those cases. Some approximate approach would be great but it is a significant bit of work.
So, no such option, sorry.