How can I add remote repositories in Mercurial?

You add entries to the [paths] section of your local clone's .hg/hgrc file. Here's an example of a section that would go in the .hg/hgrc file:

[paths]
remote1 = http://path/to/remote1
remote2 = http://path/to/remote2

You can then use commands like hg push remote1 to send changesets to that repo. If you want that remote repo to update is working directory you'd need to put a changegroup hook in place at that remote location that does an update. That would look something like:

[hooks]
changegroup = hg update 2>&1 > /dev/null && path/to/script/restart-server.sh

Not everyone is a big fan of having remote repos automatically update their working directories on push, and it's certainly not the default.

if you want to add default path, you have to work with default in your ~project/.hg/hgrc file. As Follows:

[paths]
default = https://path/to/your/repo

Good Luck.

You could have a look to hg-git GitHub plugin:

adding the ability to push to and pull from a Git server repository from Mercurial.
This means you can collaborate on Git based projects from Mercurial, or use a Git server as a collaboration point for a team with developers using both Git and Mercurial.

Note: I haven't tested that tool with the latest versions of Mercurial.