You need to create corresponding repositories on Bitbucket for your subrepos -- subrepositories are just normal Mercurial repositoires and since you cannot create repositories on Bitbucket by merely pushing, you need to create the subrepos first.
Note that you will have to use subrepo paths that makes the source paths of the subrepos siblings of the main repository:
foo = ../libfoo
bar = ../libbar
That means that you will need three repositories (main, libfoo, and libbar) on Bitbucket.