Apparently, you can import .xwb files (XACT wave banks) to Microsoft XACT audio framework and used for a new or existing project (.xap). This seems to be only way, how xwb to xap conversion could be achieved as we haven't found any other information saying otherwise.
Convert XACT wave bank data to XACT project file.
We found one software available in the database for this conversion.
Microsoft Windows software - convert xwb to xap on Windows
A framework for making audio resources
Convert from xwb 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.