The RadEditorMOSS.wsp is a normal SharePoint solution package. This means that it is a simple CAB file. You can rename it to RadEditorMOSS.cab and open it with any archiving software that supports windows cabinet files.
If you want to modify the RadEditorMOSS solution, you will need to extract it and then package it again using the makecab windows utility. To use Makecab, you will also need to create a .DDF file, which lists the files to be included in the cabinet file.
All the best,
the Telerik team
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