The command Send Email code isn't designed to accept the Username/Password from the WFA Credential page. If you look at the command code, it just takes what you have provided as the parameters. So with this certified command this isn't possible. A user may created his/her own command to do it, but that would need some work.
Regarding storing the Password in text format, I agree its a security bug and must be corrected. I should file a bug for it to be corrected perhaps in the next release.
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