Audacity can export in IMA ADPCM format.
In the export dialogue screen you need to select “Other uncompressed files” as the file format, and then click the “Options” button to set “WAV” as the “Header” and “IMA ADPCM” as the “Encoding”. See here for more information: http://manual.audacityteam.org/o/man/file_export_dialog.html
To make the exported file have a sample rate of 8000 Hz, you need to set the “Project Rate” to “8000” before you export (bottom left corner of the main Audacity window.)