Stash is where you would store your remote repo, while SourceTree is a GUI frontend for the git command line, meaning it's where you manipulate your local repo.
Therefore, you can clone a repo from Stash to SourceTree - this really just means using SourceTree as the client with which you clone a repo from Stash to your local computer. Then you make changes locally and commit them. Then you can use SourceTree to push the new repos back to the remote repo. You do not use the clone command to send data from the local computer to the remote.
I don't use stash myself, but the basic idea here is going to be as follows:
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