Not that I know of. No workarounds. You need to use DAX measures instead. Other than that it's a new language, you should be able to do just about anything you need in DAX.
I haven't tested what happens if you build a workbook in PowerPivot in Excel 2010 with MDX calculations then upgrade. Feel free to test. But that's not a good scenario since even if it worked, you would never be able to modify those MDX calculations again after upgrading.
I haven't tested what happens if you build a workbook in PowerPivot in Excel 2010 with MDX calculations then upgrade. Feel free to test. But that's not a good scenario since even if it worked, you would never be able to modify those MDX calculations again after upgrading.