Short answer: no, it's not possible to login as another user via OAuth and the Mobile SDK.
The closest thing you could do would be to have your app request
refresh_token scope (as well as whichever other scopes it needs); user A could then login and the app would pass user A's refresh token to user B, who would create an access token from it. The app should allow A or B to revoke the refresh token, particularly since they are a limited resource (5 per user per app).
Note - I haven't done any security analysis on the above. It allows user B to impersonate user A as far as the scope of the token allows. You should carefully consider if this is what you want.