Open Mathematica nb with Sage

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This is almost certainly not possible, since Sage is not a Mathematica clone! (It is not like the situation between Octave and Matlab.) You can only upload Sage worksheets (and certain other related formats like .rst) to the Sage notebook. That said, at least some syntax is similar to some Mathematica syntax, so you may be able to convert fairly easily if you are mostly using simple commands.

If you are looking for something of a clone, Mathics may or may not fill the bill. You can also look at this question which you may find helpful. Good luck!