There aren't parametric tools to edit a STEP import.
You can use the shape as a basis for offsets.
Make an assembly with the part model in it and then create a new component in the assembly. Make the new component active and the features you create will be in the new component. The part will be available for reference. Search Google for "create component in proe assembly" There's at least one Youtube video.
Alternatively, make a case and just adjust the dimensions to fit the tablet model dimensions.