You can simply resave .mcd documents in MathCad to a .xmcd worksheet should you need it. But, if you do not have access to MathCad it will be very hard (if not impossible) to perform this conversion without the program.
Convert MathCAD document file to PTC Mathcad worksheet.
We have found one software suitable in our database for this conversion.
Microsoft Windows software - convert mcd to xmcd on Windows
A mathematical and engineering calculation software
Convert from mcd file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.