Yes, you are able to use ReFile to add a dependant to your 2018 income tax return.  Please see the following FAQ for more information.

https://turbotax.community.intuit.ca/replies/3639538

I did not enter the dependent information in my original return. When I entered the dependent information and refile the return, neither the "ReFile change Summary" or the "Refile Information for CRA" forms contain the new dependent's information. It only show changes to the DTC transfer. Does Refile actually submits the dependent's information to the CRA?

Does Refile send the only the changes (as in the "T1- adjustment form") or does it send the entire tax file with the changes?